[JRTJH]

If you have a "water control center" on the trailer with city water connections, dump valve handles, etc, then you may find the water heater bypass control in that panel. If you don't have that type "water control center" then the water heater bypass valve is located on the water input (bottom water connection) at the water heater. There may be a 2"-6" pipe nipple on the connector, but there's an "cold water input to the water heater", a "bypass pipe that usually extends up to connect with the hot water outlet at the top of the water heater. The valve is a "quarter turn" valve and aligning the handle with the vertical pipe places the water heater in "bypass" while turning it "90 degrees from that position" will place the water heater in "normal operation".

The quarter turn valve is very easy to overlook, isn't really much bigger than the half inch PEX to which it's connected.

Here is a photo of a typical bypass valve in the "normal" position on the rear of the water heater.