Since you have an inverter.. This is a line drawing of how most motor homes with INVERTERS are wired
Inverter-~--Battery--Circuit breaker---Rest of 12 volt system including furnace
ON the other hand if you did not have an inverter it would look like this
Battery--Breaker---Rest of 12 volt stuff including both converter and furnace
What the problem is
You have one of two problems,, Eiteher you are drawing a bunch power on the 12 volt side with lights and such and when the furnace kicks in it trips the breaker due to overload.
or the breaker has gotten Weak with age, and what way say an 80 amp breaker is now a 30 amp breaker and see above paragraph
By the way, in the first line drawing ~ is a fuse.
