My rig just uses a self enclosed usb rechargeable water dispenser designed for a 5 gallon water cooler jug. A quick online search will bring up a bunch of options from $10-25. They can be mounted on anything and draw water through a dip tube (would recommend getting nicer silicone tubing off the bat though, 6mm ID). I felt this was the best compromise for my needs as something inexpensive and fairly simple while still being very functional and decent looking. It just pulls from a 5 gallon fresh water can under the countertop, dispenses into a stainless bar sink, and drains into a second can designated for gray water.

I don't know how durable it will be for the long term, but it's cheap enough to not be too concerned about it. You could also leave enough slack in the supply line that it could be pulled outside the van and used as a low flow shower head to rinse off or for washing off sandy feet, etc.