Typically every inch of "blind" space in motorhomes are used. For my coach, part of the underside of the bed is used for the huge storage area in the basement, and the other part is used for the water pump and 40 gallon water tank. There is also AC and DC wiring in that area you would have to contend with.
Other coaches may use that space as internal storage only, and you may be able to re-utilize that. It will vary from model to model.
The larger size coach you go, the more likely it is that you can accomplish what you want to do, as there the need to stuff the functional equipment into every inch of available space diminishes as the size increases.
But then again, larger coaches tend to have larger water/black/grey tanks, so they may not be a lot of room in them either. It will vary by specific model.
A toy hauler might be your only real solution. My wife asked me the other day if we should get a toy hauler so we can put a shop in it. In reality, I think she just want's her garage back.
The only thing that works on a RV is the owner...