[hypoxia]

I was able to acquire a copy of the rear load center diagrams which show all three UREA DEF Line Heaters power come from F35 20A fuse through K35 CM1-R-12V Relay .  I replaced that relay on the assumption that it may be intermittent.  I have travelled around 700 miles without further problems.

Later model Newell are different with a relay for each line heater and probably variants of that FWIW.

Newell tech support has been outstanding!