For all the faffing about why not just set a second ( small if you have one ) tank up in the van and just use it for chemical application
If a wfp pump, say shurflo costs £80 and u use it for 6 months surely it's paid for itself.
Let's just say, u don't have much room in the van, so let's convert a wheelie bin, remove wheels, cut down to size, drill hole for outlet, and fit one, and hey presto, rig a bit of hose up and fit a pump, run wires to battery.
If it only to be use every now and then and don't want it fitted in, set wheelie bin up with pump, and put it next to van, run wires to battery.