You stated that all outlets work on the 20 amp breaker. If that's the case, its not the inverter, if you even have one.
An inverter inverts 12 volts DC into 120 volts AC, when the RV is not plugged in.
To check for an inverter you need to follow any large battery cables that may go to it.
I don't think they were to popular in older Class C MHs.
I installed one in my 1999 Class C. I had to add some battery capacity, since it only came with one house battery.