Hi,
If you put a 50 amp minimum service to the RV barn, then you can have a distribution panel, and have some 20 amp recepticals, some lights, and still have a 50 amp plug to plug in this or some other RV sometime. You might want to have a 30 amp either for this RV or if you have friends visit.
The 20 amp will come in handy when you want to run a weed eater, or Christmas lights.
What I would recommend is a 100 amp sub panel in the RV barn, then if you want to run all those things at once, they will not overload the panel, or cause excessive voltage loss.
I had not thought of a slop sink, but it sure would come in handy while washing the RV, or anything else around it. Dumping the bucket of soapy water where animals will not get to it, a great idea. Having just cold water will be acceptable. If you really needed hot water, you always have the house, or possibly start the RV heater for a while. It would also come in handy while washing your hands after pulling weeds, or something like that.
Dump station would be nice.
14' tall door so you never need to worry about scraping off your antenna or A/C while pulling in. Might not be your RV, but perhaps when selling it, the next buyer might have a 13'5" tall fifth wheel and willing to pay more for a home with a tall garage.
You might also consider a electric heater. Should you live in a area with infrequent ice, or want to take a trip in November or March, and get sudden snow, you can warm the garage to 45 and not worry about winterizing the RV.
Fred.