RKL,
I was just looking over the plumbing prints for the water heater. You might already have these, but here's the link anyway.
Plumbing Diagram 36GD
Plumbing Callouts
The only check valve I can see is V6 which is located at the water heater shown in detail view F on sheet 5 of the print.
I hate to say this, because I know how hard it is to get up behind the rear of the water heater, but that's where I think your problem is. As a last resort, before tearing apart the water panel, you might be able to look down into the water cabinenet by removing the bottom drawer in the bedroom chest of drawers. There is an opening under the bottom drawer leading into the back of the water cabinent. Using a mirror and flashlight you can probably see if there is a check valve present as the Winnie tech told you. According to the dwgs, though it should be at the water heater.
Let us know where you find it, as if the dwgs aren't correct, it be handy to know exactly where the valve is.
Joe
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Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV