Digger, you don't mention what model of Thor that you have. On my Vegas, the check valve is part of the water pump.
First try what has been suggested. Unhook city water and run off the water pump for 10 minutes. A lot of times, that will clear any debris in the check valve.
If that doesn't work you have two choices. If you are under warranty, contact Shurflo and they can supply you with a new check valve. Or you can just buy it. The other (quicker) choice is to replace the pump.
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