Thanks and kudos to Rubicon, Brian B-P and Jon V especially for the clear and concise instruction on hot water heater use.
I learned from them that the option of 2-way operation meant the following (assuming I understood their earlier posts):
-the outside-the-trailer switch controls the electric heating;
-the inside-the-trailer switch controls the propane heating; and
-operating the two simultaneously is also an option.
Also, on the propane switch, the "FLT" light means it's attempting to fire up.
Makes perfect sense, once it's explained.