12 V Water Pump

I know a few of each. The ones underneath have a stone guard fitted over the top for protection.

Mine will be fitted near the Gas bottle. Mine is an open area , not a locker , so I personally dont have an huge concerns. My gas bottle is removed when camping.

Howard,
Im assuming the water tank is mounted under the frame. Personally, i would mount it as close to the tank as possible, reason for this would be distance. Less travelling distance before the water passes through the motor. Longer it takes for the water to pass through the motor means that the motor is revving its guts out sucking it in thus causing motor to cark it sooner than it should.
Thats just my 2 cents worth.
Ange

If it is an inline pump place it as close as possible to the tank outlet with a filter before the pump to stop any solids or other from fowling the pump.
Just a simple filter that you buy for the garden sprinkler system would do.
Make sure you protect them from and flying stones etc.
To save water have a momentary switch that must be held in to operate the pump. That way one only gets the water that one needs and the pump cannot be left running and mt the tank.

Remember, if you fit it near the gas bottle, that you are responsible for that installation .... forever!
So if the next, or subsequent owner/s get injured then you may well be held liable.
I think it would be foolish to locate it near electrical power, as already said, put it close to the tank, & if necessary fit a stone guard.

Howard I have just done my camper and I put mine in the box on the draw bar I have a aqua jet duo Johnson with a 2 litre suspension tank pumps 38 litres per minute which we run through a hws and to a sink and tap. these pumps are a bilge pump which are self priming mainly used for marine purposes .they are basic and take a fair bit of punishment but they do not like foreign mater getting under the seats so as suggested above put a filter on it . mate put it where you can fit it with out the pump getting damaged it will self prime .the only thing put a switch on the pump so you can turn it of when you are not using it ,for it will draw some amps as they will cut in and out while they maintain pressure. Even the smaller pumps get a bit savage on your batteries if left on dags

is there any reason not to mount a submersable pump in the water tank?

If mounted above the tank and near the gas bottle, is it a self priming pump?

Does it need mounting lower for priming?

All pressure pumps are not waterproof due to thermal overloads (the motor bodied expand and contract when they get warm with use). They must be mounted in the driest place possible. Also Johnson pumps are not US coastguard approved for spark, so I would not mount it near a gas bottle. Marine grade Flojet pumps are US coastguard approved though. All decent sized water pumps have a 2m head and can push/pull water up to 10 meters so you do not have to mount it right beside the tank etc
Tim