Have you considered a used school bus instead of a truck?

Old buses seem to go fairly cheaply still.
One of the medium sized ones might meet your needs, be wheelchair accessible and not require any special license or storage.
Rip the seats out and you have what amounts to a truck on a 2 ton or 5 ton frame with the bonus of windows and through access from the cab.
They are also big enough and heavy enough for holding tanks and a washroom to be fitted.
(they tend to be better maintained than rental trucks too)