Other than the technical side of doing this correctly and being in the wrong sub-forum, what do you have for an RV and how long is it? TTs for ex. are poorly insulated and a heater can end up running non-stop just to keep it at a minimal low temp inside. I installed a permanent 1500 watt heater in our first TT which was only 20' long and it ran continuously and drove our power bill way up.
If you want to heat the interior to reduce the humidity level, why not use a dessicant dryer(s) or electric dehumidifier. If you use an electric heater, you also want to crack open a window and vent so moisture can escape.
For lights, do you mean the 12 volt DC lights running off the converter/battery? If that's all you need, could run an extension cord to an ordinary 15/20 amp receptacle.