I would think it all depends upon what you plan to use the garage for, how much $$ you want to spend, and your location.
If just for storage of the rv, you really don't need all the extras....a simple 50amp service panel will power outlets, lights, with enough left over for 30amps for the rv to keep the batteries charged.
If you plan on using the garage for other things, then design in the items you will need.
What is the going $$/sq ft that contractors are charging in your area? Don't know? Find out. Use this figure to determine the "max" size you can afford. At a minimum I would go with 50x20...absolute minimum. 14'x12' roll up door...16' walls.
Your location and the time of year you will be storing the rv will determine if you need to insulate or not...or even installing heating and/or AC.
As previously mentioned, during excavation, put in empty pipes for future requirements.
Once you have come up with a plan, have someone draw up some plans and get the building permit. Then make copies of the plans and ask at least 3 contractors to bid on it. Choose the contractor that you think will do the best job for the least amount of money. Always make sure that the contractor will be open for changes/additions that may come along....keeping those at a minimum of course, due to also having to change your building permit. $$
Good luck
Ron