The fuel tank is smaller than original, to make space for the lowered floor, but otherwise uses the standard pump and gauge unit. If there was an access point from above, that'll now be under the carpet, which may be more difficult to lift than in a standard vehicle.
3yr-old fuel will have lost a lot of its volatile components by now, and the remainder will have started to turn to gum. The less there was when last used, the more likely to dry up and turn to gum.
First thing to do is listen for the pump to run when the ignition is turned on. It should run for 5-10 seconds. If it does not, the electrical feed needs to be checked, but the issue is almost certainly just old fuel.
The tank needs emptying, the pump removed and cleaned. New fuel is a good solvent for the old, but needs agitation, rather than just adding more and waiting for it to dissolve the old. Once cleaned and running, the old fuel can be added to the new, if the vehicle is to be used, it'll dissolve and work ok. Otherwise disposal can be difficult.
