Sry for getting back to this thread so late, I haven't had a chance to look at it until tonight. I did as richinny
and otherssuggested, and tried plugging in the converter directly to the shore power. Worked as it should, thank God.
So yes it is just the GFCI that is getting tripped, I guess because the new converter is leaking power. My batteries were really low so the converter was probably sucking and leaking enough amps to trip it. I also read online some other people who have had similar problems in the past with the same converter. Some had tried to replace the GFCI breaker and it seemed to work at first, but later tripped again when the batteries were low. So for now I'll just run it from the outlet on the 20amp non-GFCI breaker that is on the wall near the front Gaucho. It isn't ideal, but I can get a plug that is flat and doesn't look too bad running down the foot of wall until it goes behind the gaucho, but I feel better about that then trying to rewire the outlet behind the gaucho. The converter has been running strong on that circuit now for about 25 mins without problem, so hopefully that fixes the problem. Also, I don't feel a need to replace the existing GFCI breaker as it has been working just fine for other appliances, etc
I am also going to replace my batteries as they are about 6 years old now, and I don't know how they have been treated, they seem like they have been both overcharged by the old univolt and also completely drained too many times. I don't have a lot of experience with RV's but based on boating knowledge they seem like they drain a little too quickly from full charge.
Thanks to everyone for the great suggestions! Was really worried I would have to some crazy circuit testing and rewiring to get this all working. My perception that the universe is a simple place has returned to normal denial... 8)