Some of the wheelchair vehicle adaptors cut the fuel tank down to make room for the dropped floor. Reduced capacity, but otherwise pipes, pump and fuel gauge all still work.
Most of the recent WAV manufacturers are still in business, so if your tank is 'special', contact them for a replacement.
As we cannot see the vehicle, we cannot guess who might have done your conversion. Gowrings usually put a badge on the tailgate. Others do similar. Somewhere in the vehicle, possibly a doorshut, or in the rear area somewhere, should be a plate with the converters details. These vehicles get type approved, so should have identification somewhere. The V5 may have details.