Put one of these on the outside of the camper. It's a male 110 plug that the end of an extension cord plugs into. From there, add a circuit breaker or fuse and run your #12 (20 Amp circuit) or #14 (15 Amp circuit) Romex wire, using a GFCI outlet for the first outlet in the circuit (protecting the other outlets down the line). I did this to my cargo trailer, but I omitted the breaker/fuse for simplicity, relying on the source breaker to trip if something's wrong. In 12 years, I haven't had an issue, but I don't run 1500 watt hairdryers in my trailer, either.
I've encountered a few campgrounds that do not have 20 or 15 amp service. Only 30 or 50 Amp. That's a bit of a snag for this plan. Adding a breaker to your existing panel is probably easiest, if there's space and capacity available. I'm not an electrician.
This is an example. I made zero effort to find the best deal:
http://www.amazon.com/Leviton-5278-CWP-Receptacle-Industrial-Grounding/dp/B003ATXIBG