I don't have a Toy Hauler, but I use a 15-20 amp extension cord from the power post all the time. I just put it through one of he windows. Pull the screen out a bit leave the window cracked open. Besides .... ESPECIALLY in the winter ... we always leave a couple windows cracked anyway to reduce inside condensation. This serves 2 purposes!
I use the extension cord when cooking with our electric skillet or electric griddle. When cooking outside, I use the same extension cord, just pull it out of the window.
A few times, I've used it for a ceramic space heater, but finally figured out we can run 2 heaters at the same time plugged inside the camper. But the electric skillet just pulls a bit too much.
AND! I've NEVER had anyone from any campground tell me I cannot or should not do this. If the campground electric post has a 30 amp for the camper and 2 - 20 amp household plus .... why can't they all be used?