First of all, most people will only use a 1000-1500 Watt Inverter. Using a really high powered one will suck your COACH batteries down fast. Most motorhomes will only use an inverter to power their TV's, and maybe provide power to a receptacle near the dash so a computer can be used while going down the road.
So, the first thing I would check is that I have a really good Ground connection on the 110VAC side to the motorhome chassis. Remember, a GFI breaker senses any low current leakage between the COMMON wiring of your RV and the actual ground connection.
Therefore, assuming your previous inverter was wired right, I cna only suspect your temporary wiring.
Check this out.