My slide has storage under bed that moves with the slide. I agree with you that it would have to be stationary either on the floor or on the slide.
Probably doable to retrofit but not necessarily "easy".
Brian will have to comment regarding this. I would think the bed support structure would need some modification to fit the replacement tank in and move around it.
Also, some form of heat will be needed in cold temps. I have experience here
Maybe the tank heater switches will actually do something. Maybe heat the bed!