While some truckers I know prepare their food ahead of time, this works well for people on dedicated runs, those who run OTR need to be able to put things together from ingredients they have available.
Some standard types of items to keep in your truck would be things like spices you like and canned items that can be heated up. But you are not relegated to simply eating reheated foods even if you aren't a gourmet cook.
When you choose your microwave for your truck, you need to keep in mind how much power it is going to draw and make sure to get an inverter that will handle that much power. (You can check out this article about installing the microwave here). How much power/wattage your microwave had will be the biggest obstacle when it comes to choosing what you are able to prepare in the way of food.
The lower end microwaves don't have enough power to pop popcorn or heat water sufficiently to make things like minute rice or boil noodles. Getting a microwave and inverter that can handle heating for several minutes opens up your menu options considerably.
You might also want to invest in a microwave cook book.
If you are only cooking for one person, heating up meals is perfectly fine. Make ahead meals can be frozen, or at least portions of them can be frozen, then reheated in the microwave for a healthy meal on the road. We all know that trying to eat on the road, outside of what you have prepared is a) expensive and b) really not good for you.
Here are a selection of microwaves that you might consider for your truck:
