With the garage being 100 feet away you should be using wire greater than the normal 10 gauge for 30 amps. The wire will probably be in conduit so go to a 8 or 6 gauge wire run from the garage.
This is not considering the length of wire from your new box to the RV.
Voltage drop
100 feet, 10 AGW wire at 20 amp load is 4.1 volts
100 feet, 10 AGW wire at 30 amp load is 6.1 volts
100 feet, 8 AGW wire at 20 amp load is 2.6 volts
100 feet, 6 AGW wire at 20 amp load is 1.6 volts
Use a local ground pole.
wiring it up, select 30 amp service on the left.
http://www.myrv.us/electric/
