[hankpage]

If I am reading the markings on that breaker it is REC/CON or receptacle/converter. As Festus2 mentioned we are limited to what we can use at the same time. Especially on the same circuit. With that breaker off, try to find another outlet that works on a different circuit for your heater. Your converter is supplying 12v for you lights, furnace and other controls. That combined with your heater is most likely more than 15 amps.
I have owned three trailers with 30amp service, only one of them would allow me to use A/C and electric water heater at the same time.. It is just the nature of the beast. WH + A/C + Converter = close to or over 30 amps. JM2¢, Hank .......... Welcome to the forum.

"As the title says when I'm running ac and the microwave at the same time it trips a 15 amp breaker everytime."
The A/C and micro are both on separate 20amp breakers???