ECC MODULEOne of the really neat things about the D (in my opinion), is its ability to run both air conditioners on only 30 amps.   We travelled with friends that had a "50 amp coach" - and if they didn't have 50 amp service, their rear roof air would not work! Needless to say, this became a real issue in the midwest this summer!
The device that makes this possible in the D is something called ECC module that is made by a company called Intellitec.   Failures of this unit have been somewhat rare, and Ed Kennedy has supplied the following great information on this unit.
NOTE: This is the unit that is also responsible for the way your air conditioner operates - the fan stays on until the room temperature is 5 degrees below the thermostat setting.   Personally, I hate this noise! If you want to do a simple modification that stops the fan when the compressor stops, click here for details!
You should have a black box with yellow lettering on it behind the main
electrical panel.   It is the Intellitec ECC module which controls the
shedding of power between the front and rear ACs.   (Photo available if
needed) There is a test switch on one side of it, between the connectors
going to the ACs.   Per Intellitec: "The TEST switch is a three position
switch located on the center, front edge of the control box.   It is marked
TEST on the box label.   The normal setting of this switch is in the center
position.   Moving this switch to the left will operate the front air
conditioner continuously in the HIGH COOL mode.   Moing the switch to the
right will operate the rear air conditioner continuously in the HIGH COOL
mode.   If the air conditioners do not operate with this switch, the problem
is with the air conditioner or power being supplied to it.   This switch can
be used to manually operate the air conditioners in event of a system failure.
The SHORT indicator is an LED located on the edge of the control module.   If
the system is not operating properly and this indicator is on, there is a
short in the wiring to the air conditioners."
I found that Intellitec, 131 Eisenhower Lane North, Lombard, Illinois, 60148,
1-800-251-2408 or 708-268-0100 was very helpful when my control unit went
south and did what you are describing.