I always get confused whether "chassis" is intended to mean engine or house.
The purpose of the converter is to charge your house batteries. You can charge your engine battery from the house batteries by pressing the dash button with the picture of the battery. If you are plugged into shore power (or generator is running), you will be able to charge the engine battery with that button more efficiently. The button energizes the Intellitec relay. The Intellitec IRD was not intended for this, but it works. It is a handy way to get your engine running if the engine battery is dead, or to get your generator running if your house batteries have crapped out.
This
Intellitec IRD (Isolator Relay Delay) system will automatically charge your house batteries after some charge is applied to your engine battery. It is called "split charging". It uses the engine alternator, a high capacity source of 12 volts.
On to the GFI tripping problem. Your block heater may be bad & the GFI is working properly (to save you from electrocution) Or, perhaps you need a conventional (non-GFI) outlet. I think I would 1st ground your coach (frame to electric panel), plug in engine heater to an extension cord from shore power to see if breaker trips. If not, I'd disconnect the power cord & the ground wire, connect an AC voltmeter to electric panel and to coach frame & repeat the experiment. If you see anything more than a couple volts, you've got a problem with the block-heater (or something else) that needs to be addressed.
Measuring voltage to earth-ground isn't valid when using your generator. However, the use of a GFI could be a problem regardless of the source of the AC power.