Adding 20amp plug

In my 2007 Winniebago Voyage there is a small access port in the outside storage just to the left of the entrance steps. It looks like you could run a heavy extension cord through it. On cold evenings my wife likes to run a small 110 heater. If we only have 30 amps, this can cause a breaker to throw. Has anyone added an extra 20 amp indoor plug powered by an extension cord running to the 20 amp plug in the campground power outlet? There is always at least a 30 and 20 in every campground power box. Before I get out the sawsall, am I missing something stupid or dangerous?