I've seen and been a part of the construction plan the OP intends. It sounds like he may be building the RV garage and maybe the house at his own schedule. It certainly can be done and for some folks, is the only way to accomplish it with the budget available.
As far as living in the 5'er and it parked inside, that is totally doable. It will just take some planning for utilities and local weather conditions will be the task manager for that. There is an indoor RV park in ND that caters to the oilfield crowd. They only rent to pull-behinds and they have the sewer, water, and propane worked out.
Depending on where this RV garage will be erected and local weather conditions, in ground heating is a good solution. Once the boiler, the pipes and water, and the concrete is all heated, it can be the best for a garage/shop. If the building is well insulated, there wouldn't be a need for excessive heating or dealing with condensation in the RV. A couple of electric heaters in the RV and run the in-floor to a moderate temperature, a guy could have the rest of the building for a shop or store materials for house building.