Wiring to code is very doable, just make sure you use heavy enough wire and I think tagging along with the microwave is a good idea and sounds like your quickest and easiest choice. All connections must be in a box and you have to be able to get to them if needed , as previously stated.
An RV electric furnace insert is something you may want to consider if you plan on keeping your camper for a long time (actually you could remove it and transfer it to another camper if you wanted)
I plan to get this next year (I have spent enough money this year LOL)
Also you could look into a heat strip for the inside of your A/C unit, many A/C units have a place inside already wired to accept this ad on, you then could run the A/C heater all night without doing any other major wiring. (It basically turns your A/C unit into a big hair dryer)
(I am going to look into doing this over the winter, If I am able to install this into my A/C unit and it gives off a fair amount of heat, I will forgo the Electric furnace insert)
Here is a link to a heat strip, it is an example of what they are, I don't know if it is exact fit for your camper, I am just showing you what they look like.
Dometic Brisk 2 Air Conditioner Ducted Heat Strip | RV Parts Country
I don't know if there is room in the Electric panel to ad a separate circuit for a heater but if there is it would be something an electrician could do or guide you on if you are handy.
Keep us posted and let us know what you do.