Thanks all! I appreciate you all taking the time to answer my questions!
It's good that your shore power system is new, or at least in good condition.
Just remember that while running heaters and appliances, you should try not to exceed 80% of the maximum rating for an extended period of time.
Otherwise you get a syndrome called "boiling the frog". The wiring could get hot, and have a meltdown without ever tripping the breaker. If you're running two heaters, you'll need to shut off a heater momentarily to run your microwave oven or other appliance.
I'm not trying to turn you into an electrical engineer, just do a little simple math in your head:
You have a bucket of 30 amps to work with:
80% of 30= 24 amps safe, indefinite working load.
1500w heater. How many amps? 1500 / 120= 12.5 amps.
2nd heater set to 900w. How many amps? 900 / 120= 7.5 amps
12.5 + 7.5= 20 amps.
Mini Fridge: 6 amp start-up surge, 1.5 amps normal running load.
So, you can safely
run one heater full-blast, another heater on 1/2 power, run your mini-fridge, and still be safely within your limits, and not experience any "nuisance tripping" of your circuit breakers.
You can always figure out your electrical loads by reading the labels on the backs of your appliances. They'll always give you something to work with: amps or watts. A casual manipulation with Ohm's Law, and you'll know if you're going to overload your wiring or not.