Here is check valve you are talking about. It on the top back of the water heater. It is brass screwed in directly to the heater. Looks like this.
Here is a replacement for it. I bought them here the last time as they were cheaper at that time, pre-covid. You can look at other places for the same check.
Valterra Check Valve for RV Fresh Water Systems - Dual 1/2" MPT - Brass - P23415LFVP
And yes, there have been times they that water heater check leaks backwards into an empty water tank as the O ring seal gets sediment on it or it falls off the groove. This shows up often when you winterize and antifreeze drips back into the water heater as the system cannot build pressure and the water heater is empty on purpose.
If you are sanitizing the camper, by chance are you be bypassing the water heater and have an empty water heater? Do you sanitize the water heater?
The pump cycling with "no leaks" visible with a "full water heater" normally points to a leak in the fresh water pump. Water drains back through the pump, then back into the fresh tank and you will never see any water. If you want to test that it is the pump, fill the entire system, water heater too, and pressurize up the system. If the pump keeps burping every so many minutes and you are positive there are no water leaks, then it points to the check valve inside the pump.
You can also hook up city water and if the fresh tank overflows after an hour or more or keeps gaining water in the fresh tank, the only way it can get water in the fresh tank in this way if backwards through that check inside the water pump.
Hope this helps and let us know how you make out.
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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