Thanks to Stan for supplying a troubleshooting guide promptly after an email request, and for providing a link re disassembling the pump to inspect for problems.
I have concluded that the pressure valve mechanism jammed and cracked, likely due to freeze damage (notwithstanding my efforts to drain and purge system). My bad. My risk. It appears that the damage could have resulted from freezing of a very small amount of water (i.e., a drop or two). We often use the camper for skiing and it got exposed to sub-zero F temperatures several times this winter. Next winter I will likely remove the pump before winter.
I called the pump manufacturer (Shurflo) and got the info re the replacement part, which I found on Amazon (Prime) for $21. ETA: Fix was successful and not difficult.
The pump in my Hawk/front dinette is screwed onto an inside cabinet wall, a PITA to access. I mounted it via screws from the outside, now much easier to remove and install.