You obviously have some draw in the RV, then like every electric motor the start up demand spikes thus tripping your GFI. 15 Amp at 120v does not go very far if you battery charger is on, refer is on or any number of load combinations.
Just like your A/C it requires more than 3200W to start but 1500w or under to run, your wet vac is doing similar just on a smaller scale. If possible look to see if you are using a 15amp converter plug may go to 20amp and check conductor size and see if you can upgrade to 20amp circuit and GFI if you can. 15-20 amp is significant in running basics in your RV
Get a watt meter this will tell you your loads in lehman terms
https://www.amazon.com/Poniie-PN2000...s%2C218&sr=8-3
