And of course, there is also the requirement that your existing breaker panel is not already fully loaded at at legal capacity.
Certainly, it's not often that you would need 50 amp power beside the garage. If you have a 100 amp main service, or even a 125 amp service, and you plan on running air conditioning in the rig and in the house in mid summer, you can certainly get into dicey conditions for the main service to fill.
There is a lot to consider here, and you should get a professional to check it out. 40 years electrical here.