weight distribution system

What type/brand of level riders do you have?

The VS has 3 levels of towing capacity. 1200kg 1490kg and 2100kg all of which are dependant upon level of towing equipment and vehicle equipment.
A WDH (Weight Distribution Hitch) will not increase your towing capacity neither is it a substitute for an overloaded vehicle or excessive tow ball weight.
Your problem could be related to worn/sagging rear springs or shock absorbers fatigue.
Ideally you should be able to have the ride height of your vehicle returned to normal height with suitably adjusted WDH and adequate suspension.
In order to maintain a safe towing combination I suggest you get those suspension compnents checked if you are not sure of their condition. You may (MAY) consider (air) pump-up shockers which whilst not increasing the carrying capacity of the rear axle may help to re-adjust the ride height if everything else fails to resolve it.
Regarding increasing tension on the bars, yes you can but in doing so it may place more strain on your towing equipment (tow bar) which if only 1200 kg capacity might be susceptible to breaking particularly if subjected to undulating and or rough roads which will only add more stress and strain and is not a 'hitch receiver' type (generally has a higher rating 2100kg).
Remember when you load your tow vehicle with passengers, luggage, fuel etc it will also caue added strain on your suspension components. No doubt you have seen van combinations with the rear of the tow vehicle sagging (dragging a**e on the ground). Yes many people do get away with it, HOWEVER in the interests of safety and well being of your family, the more stable you can make your combination the safer it will be for all and believe me a lot less stressful for you whilst travelling. Good luck with it I'm sure you will manage to work it out. Just take your time and don't over do ot.

I tow a 1.5 tonne boat behind my VL. I had to get a towbar rated to 1.6 tonnes (Hayman Reese WDH). I also fitted Poly Air airbags, these work a treat, keeping the back at normal height. I also use a two bar ride leveller but it wasn't quite enough for that weight.

Get a pair of polyairs fitted to the rear springs.