I've never used the hot water heater. Tinkering with it now. I cannot find the owners manual for the heater.
First identify just which water heater make (Suburban or Dometic) and model you have and download the owners manual from the manufacturer's respective website - Suburban
The camper manual states to open the bypass valve for the hot water tank. I found a valve under the sofa but cannot tell which way is open or closed.
Any bypass system will be installed on the end of the water heater and may make use of one, two, or three valves so that valve you found under your sofa isn't it. Since it's the trailer manufacturer that would install this bypass you won't find any information in the water heater manual but rather your trailer owner's manual. Provide a link to pics of your water heater and we can tell you how to set those valves for correct operation.