This is just a suggestion: completely disconnect your water heater (remove supply wires) and then plug your trailer into the GFCI again and see if it trips......on occasion the water heater element has burnt out and is tracking thus tripping GFCI....and it will not trip on a regular breaker.
2011 Rockwood 8319SS with ProPride 3P hitch/GoodYear Marathons/TST TPMS 507
2008 F250 Lariat CC SB 6.8L V10 4.10 axle ratio (NITTO TERRA GRAPLER G2 LT275/65R20E)
Yamaha 3000iseb generator:Progressive Ind. EMS-HW30C : Eastern Ontario
Nights Camped:2012(36) 2013 (17) 2014 (18) 2015 (18) 2016 (36) 2017 (32) 2018 (42)