Hi retired53,
Welcome to the Heartland Owners Forum.
The furnace and water heater both consume a lot of propane. If they're both igniting at the same time, the refrigerator propane feed may be getting starved. Once the flame sensor detects loss of flame, it'll close the gas valve and shut down for safety reasons.
Try running off one tank with the other closed. If it still happens, swap to run off the previously closed tank only.
If you have a model with propane tank on each side, the doorside pigtail and regulator are likely weak spots where propane flow might be reduced, leading to what you're experiencing. By running on one tank at a time, you should be able to isolate the problem, if indeed it's a question of sufficient propane flow.