OP you should be able to run both the fireplace heater and brew a cup at the same time on 30amp. I wonder what else you had running on electric,maybe the water heater? The problem if you were throwing the breaker could be a bad breaker and or a loose connection at the power pole.
We often run on 30amp although we are 50amp and it just takes a little power management to get by. I'll run the WH on gas just so I don't have to worry about throwing a breaker and I know that I can brew a cup and run the fireplace heater but I won't toast a english muffin at the same time and DW can't have her curling iron running at the same time.
50amp is nice when you need it, think running 2 AC's when your rig is sitting in the sun and the temps are in the 90's but it's easy to work around 30amps.