Are you still having hot water issues?
To your direct question, where is the water heater check valve? On the newer campers there is one screwed into the top discharge port. It is part of the winterizing bypass kit. It comes into play when you bypass the tank when pumping food grade antifreeze in the system it prevents antifreeze from back flowing into the heater.
See theses 2 pics. This is off a 2004 Sunline. Do not know if this if close to your model year.
Here is an angle view. The top tank fitting has a hex on it, that is the check valve.
Sometimes that check valve will fly apart inside or the plunger get stuck from mineral build up in the water. Either condition can restrict flow. The check valve is a plastic plunger with an O ring, a spring and a little plastic clip to hold the plunger from coming out.
As Tommie stated make sure the lower bypass valve is in the correct position and not bypassing the heater. We do not know if this was working all along and then stopped or this is the 1st time you ever used the camper and cannot get it to work.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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