My way is very simplistic but it is how I did my self build but it does work.
You need fused power cable to a switch which turns the pump on & off, a cable to the + side of the pump and a cable from the - side of the pump to known earth, with the switch in the on position it will work if the pump is functional.
Not seeing the van does make it more difficult.
If it has switches on the taps, these may negate the method I use, they are often going wrong and I'd never use them.
I would fit a simple Shurflo pressure operated pump, but of course you may already have one fitted and it's wired via the tap switches, get around that by simply following my method and disconnecting the tap switches and using the direct wiring above
With an old van it could have been messed with by countless people with very little knowledge due to how it was manufactured, I go by one wire = one job.