Caravan anti-sway bars.

You can, as well as a WDH buy Antisway bars but if you load your caravan correctly you should never need them.
If your caravan is tail heavy it will still be tail heavy with or without a WDH.
Any trailer or van should be loaded ON. That is to say it has more weight towards the front which gives added stability and aids tracking

" If your caravan is tail heavy it will still be tail heavy with or without a WDH." is not strictly correct. I agree the van has to be loaded correctly with correct weight distribution however the Hayman Reese WDH DOES take some load off the the caravan wheels and some load off the rear wheel of the towing vehicle and redistributes it to the front wheels of the towing vehicle. By correctly redistributing the weight the tendency to sway is eliminated and as matter of fact fuel consumption reduces also. Another added benefit is that the headlamps don't point into space either. These WDH are so effective that if you over tension the torsion bars the vehicle will not tow uphill on an unsealed road due to insufficient load on the rear wheel to gain traction.

Sorry to say you are completely incorrect . If your van is tail heavy before you apply the hitch it will be even worse after you do. Reason. The hitch spreads the weight BOTH WAYS and cant do anything else. It straightens out the rig and transfers weight both to the front of the car and back to the axles of the van.
The ball weight remains the same but is spread somewhat.

As for your last sentence it simply doesnt make sense. You cant tension them up to remove weight from the rear axle as that is the pivot point for the weight and when going up a hill the centre of gravity in the car moves back and there is more weight on the rear than the front.
If your theory worked why do we have to put heavier springs and helper bags in the rear of tow vehicles.
According to that sentence we should just tighten up the WDH till the back wheels come off the ground???????

The reason for lost traction is that the tyre footprint cant overcome the extra weight it is towing and therefore slips.

The result of over tightening a hitch is usually broken bars as it doesnt leave enuf allowance for the bars to move forward and back when going around corners.
That is the reason the manufacturers recommend 5 links to be the tightest they are pulled up. To get this you use the cams to adjust the angle of the towball to allow this.

Very sorry to say I am not completely incorrect. I suggest you read the Hayman-Reese literature on how the WDH operates. Also I have results from a weighbridge test done on each separate axle before and after the installation of the WDH and it DOES take load off the towed vehicle wheels and the towing vehicles rear wheels and transfers some of that load to the front wheels of the towing vehicle so that the rig tows level and the correct weight is on the steering wheels. I have a set of my own and have towed 2 horses in a double float (2400kg) for years so I know how it works. Before you buy the torsion bars you place some load scales (suppliedon loan by Hayman-Resse) under the tow ball and then connect what is to be towed in its normally loaded condition so you can measure the weight imposed by the towed vehicle on the tow ball. You then select the correct torsion bars, either 500, 750 or 1000kg. The bars are then fitted into the ball mount hitch and tensioned correctly. You have to imagine the torsion bars being like wheel barrow handles so you are lifting up the rear of the towing vehicle, which includes some of the downwards weight on the tow ball, and this then transfers some of that weight to the front of the towing vehicle. It does NOT Transfer weight to the rear to the towed vehicle which is an illogical proposition which if true begs the question: Where would it get the weight from to transfer to the towed vehicle axle/s, how would it do that and why would you want to?
To answer your question why helper springs and air bags are needed is because the weight imposed on the tow ball from the towed vehicle is too high. The helper springs and air bags don't change that situation, they just make it "look" better as do pump up air shockies but the downwards weight remains the same. With the WDH the pivot point is the front wheel of the towing vehicle NOT the rear as you say so helper springs and other substitutes are not needed. If your suppostion was true re the tyre footprint then how come when I reduce the tension on the torsion bars and place more load on the rear axle the vehicle then pulls the loaded float up the hill from a standing start no problem. Re cornering the links in the tension chain allow fore and aft movement. I think you will find the vehicle centroid moves a very small amount when going uphill. If you weighed the front and rear axle when on the flat then again when on the uphill slope the difference would be negligble. You still have the same components in the vehicle distributed the same within the vehicle and bearing down on the front and rear axles almost the same on the flat or uphill (unlesss we are talking a 45 degree hill)

Very sorry to say I am not completely incorrect. I suggest you read the Hayman-Reese literature on how the WDH operates. Also I have results from a weighbridge test done on each separate axle before and after the installation of the WDH and it DOES take load off the towed vehicle wheels and the towing vehicles rear wheels and transfers some of that load to the front wheels of the towing vehicle so that the rig tows level and the correct weight is on the steering wheels. I have a set of my own and have towed 2 horses in a double float (2400kg) for years so I know how it works. Before you buy the torsion bars you place some load scales (suppliedon loan by Hayman-Resse) under the tow ball and then connect what is to be towed in its normally loaded condition so you can measure the weight imposed by the towed vehicle on the tow ball. You then select the correct torsion bars, either 500, 750 or 1000kg. The bars are then fitted into the ball mount hitch and tensioned correctly. You have to imagine the torsion bars being like wheel barrow handles so you are lifting up the rear of the towing vehicle, which includes some of the downwards weight on the tow ball, and this then transfers some of that weight to the front of the towing vehicle. It does NOT Transfer weight to the rear to the towed vehicle which is an illogical proposition which if true begs the question: Where would it get the weight from to transfer to the towed vehicle axle/s, how would it do that and why would you want to?
To answer your question why helper springs and air bags are needed is because the weight imposed on the tow ball from the towed vehicle is too high. The helper springs and air bags don't change that situation, they just make it "look" better as do pump up air shockies but the downwards weight remains the same. With the WDH the pivot point is the front wheel of the towing vehicle NOT the rear as you say so helper springs and other substitutes are not needed. If your suppostion was true re the tyre footprint then how come when I reduce the tension on the torsion bars and place more load on the rear axle the vehicle then pulls the loaded float up the hill from a standing start no problem. Re cornering the links in the tension chain allow fore and aft movement. I think you will find the vehicle centroid moves a very small amount when going uphill. If you weighed the front and rear axle when on the flat then again when on the uphill slope the difference would be negligble. You still have the same components in the vehicle distributed the same within the vehicle and bearing down on the front and rear axles almost the same on the flat or uphill (unlesss we are talking a 45 degree hill)