I think the first question should be, "Which Solar Flex system is installed in the OP's trailer and what inverter type is installed?"
If it came from Keystone with the Solar Flex 200, the inverter was not installed by Keystone, but by the selling dealer. That means they, not Keystone, are responsible for the wiring. If the inverter was installed incorrectly (miswired) it could be the problem.
If the traielr came from Keystone with the Solar Flex 400, then the inverter was installed at the factory and Keystone is responsible for the wiring.
So, depending on those circumstances, the "local dealer that checked the trailer" may not have any idea about how the selling dealer installed the inverter and may not have done any troubleshooting on the "not Keystone system".....
Lots more information needed to even start to figure out the problem. So far we don't even know if it's a pass through inverter with an internal transfer switch or a DC:AC inverter with an external transfer switch or a AC
C inverter with no transfer switch but rather a plug to unplug BEFORE connecting to campground shore power (hopefully not, but).