It worked without issue earlier, that's a good sign. You have not reported yet if it still works OK with non-GFI supply breakers but it currently trips the GFI protection supply breaker. Seeing if a breaker inside the camper trips is not really going to advance the investigation much. I will walk you through the diagnostic steps I would do if it were me, without buying any new parts that may or may not be needed. (You may need to buy or borrow a DVM or multimeter.) I'm guessing, based on what you have said so far, that the breakers in the TC are NOT GFI protected.
The thing to do first is to verify that the GFI still trips, and that it only trips when the "main/AC" circuit is closed, and not when the other two circuits are closed. If that is the case, then good. We can use that in our diagnostic process.
Now, a little discussion about how the system works. Typical circuit breakers like those in your house, your generator and your TC only disconnect the "line" conductors (typically black and/or red) when they are switched off. All of the ground wires (typically green or bare) continue to be connected together regardless of which of the three circuits they serve and regardless of the position of the breakers. Similarly, the neutrals (typically white) are also all connected together regardless of their circuit. What we don't want, however, is for any of the neutral conductors in your TC to be connected to any of the ground conductors in your TC. That is what the other posters are referring to as a "bonding" problem. However, because you say that there were no issues earlier running the TC off of a GFI breaker, I feel pretty confident that you don't have an "intentional" bonding issue in the breaker box. However, a short between a random neutral conductor and ground could have developed since the last time it was working properly. This would be a great place to start. So, to test that, disconnect all power sources from the TC and turn off your inverter. Using a DVM or multimeter set to the highest Ohms range or a conductivity setting, go into the breaker box and look for conductivity between the incoming neutral conductor and the ground conductor. There should be no conductivity between these two systems. If there is any conductivity between the neutral and the ground, you will need to track down where they are connected and fix that. Note that it could be an internal fault in something that is plugged into one of the TC's outlets or hardwired in; or it could be that a random neutral wire in the TC's wiring has come loose and is now touching something grounded or vice versa. On the other hand, if there is absolutely no conductivity between the neutral and ground, you'll have to look for something else.
If there is no evidence of neutrals being connected to grounds, the next thing I would look for is a short circuit where a hot wire is shorted to ground, since you have not reported back on what happens when you are not using a GFI protected source. In a system without GFI protection, a hot wire (aka, line) shorted to either neutral or ground will hopefully draw enough current through a system to trip the overcurrent protection provided by the circuit breaker after a relatively short time, typically on the order of a few seconds. The same thing would happen in a system with a GFI protector if the short is between line and neutral (i.e., also tripping the overcurrent protection system). However, if the short is between line and ground in a system with a GFI protector, the ground fault protection system will almost always trip first because they are designed to trip within milliseconds. You can test for this type of short with the DVM or multimeter as discussed above; test for conductivity between each of the line conductors on the main/AC circuit and ground. There should be absolutely no connectivity between either line conductor and ground; even a tiny amount of conductivity is supposed to trip a properly functioning GFI breaker. As with the neutral to ground short discussed above, such a fault could be an internal fault in something hardwired or plugged into the TC; or it could be a problem in the TC's internal wiring itself.
Chances are really good that if you follow the above diagnostics steps you will solve the problem. Good luck.
