since you have to get everything up front, I would do 50 amp wiring (the extra $25 for 50 wire is more than worth it)
the 12 ga wire inside would be the same for 15/20/30/50 (I used stranded lots of folks just use household solid wire has been in use by millions of RV's)
circuit breakers would be the same 15 amp
when you get a box get one like I used for under $20
first you need to decide what you want to run and where, then we can help you out some
Everyone does it differently my thread has some info in it and this is a good read thanks to Lorna
http://www.skoolie.net/forum/viewtopic.php?f=8&t=13540
right now just run an extension cord (don't over load it though)