I guess bottled water wouldn't have chlorine in it and if left in the van its temperature would equalise to that of the tank. Also rather than changing the whole tank 25% refreshers might be less stress on the fish. Then the final 25% could be a top up with Adnams
We just pop our fish into the aquaroll that has the mains water adapter kit fitted to it.
The fish are sort of 'trickle fed' with a fresh water top up on a regular basis and seem quite happy.
They do tend to blink a lot after several weeks in there (owing to the reduced light levels I assume) but recover quickly with no after effects.
I do put an extra fine gauze filter over the pump intake to prevent uneaten fish food and fish faeces going through the taps, the filter doesn't remove 100%, but a little does you no harm.
The other thing is, Should I put a towel over them at night as the light from the fridge keeps them awake. I don't put them in the fridge. Before you ask.
I assume you leave the fridge door open to keep the caravan cool at night, I would just close the fridge door at night time and leave the windows and doors wide open.
If this would not work for you then you could move the fish to the under-bed storage area, or put them outside underneath the caravan if they are of the noisy variety.