You'd need 8GA. 6GA would be better.
You are talking 25 amps at 12V and the wire and all the connectors must be properly sized for going that distance. You'd want quick disconnects too, probably.
What you may have missed is the difficulty in dealing with a 'temporary' system setup. When you park, you'll have to mess with setting it up and big heavy wire like that will get to be a chore dealing with in short order.
Consider this...a roof mounting system for solar panels only uses several wood screws (lag bolts), screwed into the wooden rafters all RVs use to make the roof. When you sell the RV, just have those few holes patched. You could do it easily with Eternabond. The wiring usually is taped to the roof and makes it's way down to the battery compartment via the refer vent. Again, easily removed, or cut and left in place.
