Hot water heater purge issue

[MarkoPolo]

We took our new 2922KD out for it's first camping this last weekend. What a great upgrade from a tent pop up!
I had a strange issue with the water heater/water pump. We did have shore power but no water. So, I filled the 20 gallon tank and opened all the hot water faucets to purge the air. Everything is working fine.
The next morning the water is at a trickle and I hear the water pump running. So I figured that I really didn't completely purge the air, so I again opened the hot water, got air coming out and even opened the pop off valve of the water heater to purge some more. Added more water, everything works properly.
That night I wake up hearing the water pump running. Again water turn on reveals only a trickle. So purge the hot water faucets, water pump stops.
The next morning, same thing happens again. I add water, purge the hot water lines and carefully check to see if water is leaking somewhere under the trailer. Working normally now. It's the only thing I can think of that might let air get in the hot water heater!
At any rate, found no leaks. By now it is time to go home. I call The Car Show and Will has never heard of anything like this. I know it doesn't make sense to me either. The only thing I can think of is that I have not purged the air properly or completely.
I intend to put up the camper in the yard and try all this again as soon as the snow melts, but thought the forum would be a great place to see if anyone else has had a similar problem. Thanks in advance.