adding fireplace

BTW: The cabinet under my TV came with a receptacle in it that is the only outlet on the breaker for that circuit. I hope you're lucky enough to have the same setup.

Excuse me! RVs may be covered by the NEC but that doesn't make the backstab outlets without an enclosure any safer to use. They're just a piece of cheap junk unfit for use anywhere. A $0.50 cheapo duplex outlet in an enclosure from the one the big box store would be much better and safer way to go - especially for a high draw circuit.

yes I see no reason it can't be done. However, my understanding of NEC code indicates several things to consider.

1) since it is a dedicated appliance it needs either it's own breaker or combined with another load and a load shed device. That is when the heater is on all the other loads are off. AC would be a good circuit for the load shed combo.
2) NEC code limits the number of breakers on a 30A panel (5?). Notice that's how most 30A trailers are wired. So in theory you are violating NEC code by adding additional breakers. That said, it agrivates me to no end to have the charger combined with the other 120V circuits, so I violated code and added another breaker to put those two on seperate breakers.

If it was me I'd add another breaker for the fireplace and not mess with a load shed device.

We had a fireplace in our last motorhome. Of all the appliances in the motorhome, the fireplace got the most use. We used it almost every night to keep the chill off, Really miss it now but not willing to give up the storage cabinet. If you can find one with HI/LOW (800 watts/1500 watts) heat, you'll have no problem running on 30 amps. In reality, you can only draw about 25 amps without heating up the circuit breaker and causing it to trip. We always opt for a 30 amp site because of the cost difference and have no problem. We'll run on electric water heater and one space heater on low with no problem. It was hard to train the wife to turn off the water heater if she wanted to used the microwave or her hair dryer but (most of the time now) she remembers and we're okay. If it's a colder night, we'll turn off the water heater and turn on two space heaters with one on low and the other on high and we still have enough power to run the refrigerator on electric and keep snug using our electric blanket. We've got an inverter/charger I set as if we only have 5 amp service. It takes longer to top of the batteries that way but that's never been a problem and the battery LED charge indicator always turns green.

I added one in the cabinet below our TV years ago. We do have a 50 amp rig, but have not tripped the breaker when on 30 amp service. Yet.

I just tapped into a nearby outlet that had little else on it and added a new outlet behind the fireplace. And that breaker has never tripped either.

Electrically, the fireplaces are usually just space heaters. If you can run a typical electric space heater, you could run the fireplace.

Assuming you want it on its own breaker, you'd have to see if there is space for a breaker or the option of swapping a single breaker for a twin breaker in your electric panel. As Dusty suggested, if you don't have room for another circuit in the panel, tying into the air conditioner circuit makes some sense as you generally would not be using both simultaneously.

I think you should be able to microwave and space heat and run the converter simultaneously if the batteries aren't really low and no other significant loads are present. It could be handy to put in a little AC ammeter panel of some sort to monitor your actual current consumption; I have installed one of these in my motorhome. I did put a low-amperage fuse in where the voltage sense line connects to the 120V system to protect the wires going to the meter in the case of a short circuit somewhere. Some of the EMS units also give a reading of current on their displays.

You are going to need about 10 amps to run the heat in the fireplace. If you just want the ambiance it takes almost no power. On 30 amp service you will pop a breaker running the microwave, fireplace and your inverter/converter but might squeak by if nothing else is running.

I put in a toe kick electric heater, It's really a small electric furnace, and hook it to the same breaker as the air conditioner, that may sound strange but you are not gong to run them both at the same time.

As long as the electric fire place uses about the same amount of amps as the ac, not more than 80% of the breaker, it will be fine.