Like everything else in life, there's no such thing as a free lunch, and if you use your microwave to heat yours, there is a certain and fearful toll to be taken. The more "shore power" your device uses, a LOT more battery power will be consumed. Let's take our "Nuke me some lunch!" scenario.
Let's say your RV microwave oven uses 1,000 watts (check out the data plate on the back of whatever device you're using) to operate. To heat up a couple of plates of last night's leftovers, let's say you'll be operating the microwave for 5 minutes. How much battery power will you "eat" for lunch?
The formula is quick and easy: Battery amp-hours consumed is this: (AC Watts/12) x 1.2 x time of use (in hours). Our microwave operates at 1000 watts, divided by 12 equals 83.3 times 1.2 (the inverter isn't completely efficient, so this factor allows for inefficiency). That equals 100. We multiply this against the usage time, 5 minutes divide by 60 gives us the "hours of use" of .083. The total amp-hours consumed in reheating lunch equals 8.3.
That may not sound like a whole lot, but if you were charging your battery with a 100 watt solar panel, it would take over an hour of strong, full sunlight to "pay" for heating up your lunch. That doesn't even take into account battery charging inefficiencies. This is one of the reasons you'll see serious boondockers running a rooftop full of solar panels.
So before you start firing up the inverter, run some quick calculations and see if you can 'pay the inverter piper.'