Thanks again to gregmchugh and others. As suggested, the problem proved to be that the Alde automatic safety valve/drain needed resetting. This involved turning the valve on top clockwise (inline with the direction of flow) and then back counterclockwise (at a right angle to the direction of flow) and then pressing in the blue button at the bottom of the unit.
I also called Alde USA, and they were wonderful. Not only did they confirm the correct actions to take, but they reminded me that, despite the demise of Roadtrek, the Alde unit has a two-year warranty that will cover any needed repairs to their unit. A sterling example of corporate responsibility.