The Roadtreks that have Tripplite inverter/chargers have had a lot of issues with tripping GFCI outlets, although normally we hear about it on post 2007 units. Of course your Tripplite could have been replaced if it is the issue. Do you know if it ever would work on a GFCI outlet? If so the van could have something gone bad.
You may want to try it in several different GFCI outlets to see if it does it on all of them, and also see if you have a Tripplite.
If it does do it on all GFCI outlets, about the only way around it is to wire in an outlet that is not GFCI protected or change to a different inverter/charger. If you chose to wire an outlet, you likely will be breaking electrical code as all garage and outdoor outlets are required to be GFCI if they are 15 or 20 amps. If you go to a 30 amp outlet it wouldn't need to be GFCI.