I'm thinking of a mini
microwave oven for my camper too. Just saw a sunbeam 700watt at walmart for $29 but didnt pick it up just yet. For that price I'm not really expecting amazing performance or longevity but should get the job done.
The big decision to size would probably be what you plan on cooking in it. In my case the microwave will be the quick cup of soup or hot chocolate in the middle of the night or when I'm stuck inside due to rain. My camper isn't going to have a stove in it because i feel if I'm camping then I should be cooking outdoors.