I fill my van tank from an ro in my flat via 1/2" hose.
I have an old shape peugeot expert, just inside of the rear doors there is an air vent and duct that went down into the bumper trim, I drilled a hole through this and have fed hose pipe through it with with a hose lock connection on the end (so hangs just below bumper) the other end of the hose is permanently attached to the tank, I have another hose coming out of the tank as an overflow and that goes down through the floor via a hole ( was already there , just had to remove a grommet)
Simon