Just reinstalled mine no problem. Had to remove it and the generator to get
out a leaking water tank. The heating element in the water heater was also
toasted when my water system drained down. I replace the 1250 watt element with a
1500 watt element from Home Depot. Reinstalled water heater, filled all four
tanks, bled the system through three HW faucets and turned it. Within 20
minutes had hot water. I do not have any type of accumulator tank.

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I had a like experience when I installed a Raritan water heater.  The system
acted like there was air in the tank and continued to do so for about 3
weeks.  With usage though I get the impression that any air was absorbed
into the water and pulled from the tank.  Today all works well w/o any of
indication if air anywhere in the system.  I do not have a check valve, nor
an expansion tank.  But then the water heater is small (6 gallon) compared
to the overall water system on the boat.

If I faulted Raritan, it is that there is no way to directly get air out of
the heater tank when filling it.  On my house, the feed / takout as well as
the pressure relief valve are all on the top of the tank.  Even with a dip
tube internal, getting air from the houses hot water tank is easy.

My experience:  Use it for about a month and see if it gets better before
doing anything other changes.

-al-