Polanski,
Welcome to the Form and Congratulations on getting your Montana.
I did a search on "check Valve"; I did click on specific wording.
This post by OZZ explains the check valve and its repair. There are many others out there that will help.
Basically there is a check valve on the hot water out side of the water heater. It fails and needs to be replaced. If you do not plan on winterizing soon you can temporarily remove it to have hot water until it is replaced.
I hope this helps.
Good luck, let us know what you find.