On most keystones there is a "excess flow device" in the lines of your system that is for detecting major leaks and all but stopping the flow of gas. Problem is sometimes when changing tanks, or opening a new tank you can get a surge and a false triggering of the shut down. I have a grill at home that does the same thing 2 out of ten times when I change cylinders in a hurry and open the valve to quickly. In case you don't still have your Keystone owners manual I will outline the procedure for you.
1. Verify both propane bottles contain sufficient propane.
2. Close both propane bottle service valves
3. Attach the propane pigtails to the service valve.
4. Close manually operated gas valves on the range and water heater.
5. Turn the indicator on the regulator to the bottle closest to it.
6. Slowly open the service valve on the bottle closest to the regulator. This is typically the bottle on the entry door side. Open valve all the way.
7. Slowly open the service valve on the bottle that is furthest from the regulator until you can hear gas start to flow through. Do not open valve all the way. Wait one minute.
8. Re-close the service valve and then slowly re-open it all the way. The system is now ready to operate.
I hope this helps and solves your problem, I always try the simple fix first. It does sound like this could be your problem because the "excess flow device" when triggered will allow no more than 20,000btu's/ hour.