[JRTJH]

Like Marshall posted, the bottom water heater port is the cold water inlet. The bypass valve/winterization valve diverts water from that inlet to the "up pipe" which connects with the hot water line "at or near the water heater outlet" which is the top port on the water heater tank.

There is a "one way valve" mounted in the top port. Its purpose is to prevent water from flowing back "into the water heater"...

I'd suspect that the one way valve in your water heater outlet port is corroded and stuck open. It is allowing the "bypassed water" to flow into the top port through that stuck open valve....

In normal operation, you probably wouldn't notice any problem since the lower bypass valve effectively closes off the "up pipe" so hot water only flows into the trailer plumbing. But, when in bypass, that up pipe delivers cold water to the one way valve and with it stuck open and no pressure to prevent the tank from filling, water flows both through the bypass system and also into the "stuck open one way valve"...

It's the brass "gadget" mounted in the top water heater tank fitting.