You should remember that with an RV, you have a 12-VDC power source in your COACH batteries.
They power numerous electrical items in the interior of your RV, including your; interior lights, Fridge control circuit board, temperature control panel, CO and Fire alarms to name a few.
Your Inverter uses your same COACh batteries to power an Inverter which provides 110-VAC for your TV's and a receptacle or two.
With your symptoms, I would first make sure these batteries are still good, have water in them and arte fully charged.
Then, I would make sure that used Inverter was a good one because it may be drawing far too much current.
And just a note, never start swapping different rating fuses. Each fuse is in a circuit it is specifically designed to SAFELY protect.