wayfun, here's what I think may be happening. Water heaters that do not have a pilot light are lit by a small electric spark. When you push the switch on, the propane comes on, the spark occurs, and then the heater flame happens. If for some reason the flame sensor does not detect the flame, the propane turns off for safety reasons. This cycle will occur several times if there is not successful lighting, and then cease until the switch is cycled. This problem can be caused by a defect, or by the propane lines not being full of propane - which is hopefully what is happening to your heater. If that is the cause of your issue, the cure is simple. When you get your trailer ready for use, turn on your propane and then go to your range top and light the burners and let them run for a few seconds until the flame is steady. This will fill your lines with propane. Then light your water heater - being sure it is full of water - and let it run until the flame is steady, cycling the switch if necessary. After that your water heater should operate normally. You may want to do this same procedure with your refrigerator and heater..
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch