A couple of things don't make sense here.
"both air conditioners worked at pdi" but don't work at your home. This would sound like the dealer's crew changed the wiring post PDI, before you drove it home.
What kind of electrical service do you have at home? You would need a 50 amp RV service (2 - 50 amp phases) to run 2 air conditioners. Do you have a power management system that manages the electrical power loads?
The 2nd thing that doesn't make any sense is "I opened up fuse panel and found a black wire that was stripped just hanging not attached to anything, wire has 120 vac on it i traced wire to rear ac unit and it is attached to neutral from ac unit." #1 Standard US electrical power code says that the black wires should be connected to the HOT side of the circuit (not get connected to a neutral wire at the air conditioner). #2 If you are measuring 120 volts AC on the bare black wire at the circuit breaker panel, what is the source of this voltage? Your high impedance input Digital AC voltmeter might be measuring the voltage from some minute electrical leakage path.
If it were me, I would first try contacting the dealer's service department with my problem, and if that doesn't get you anywhere (this should be a valid warranty claim), you could find the 2nd 20 amp breaker for the 2nd air conditioner (does your breaker box have a label identifying the breaker's functions to help find the appropriate breaker to attach the wire to? - Heartland Service can provide you this.)
Before you apply power to that rear air conditioner you need to get it wired CORRECTLY according to code. Black supply wire to black air conditioner wire, white neutral wire to air conditioner white wire, supplied ground to air conditioner ground. I would be curious to know what AC voltage you measure between the breaker box neutral buss bar and ground when everything is wired correctly. Ideally you should measure 0 volts, but if you are using extension cords to your home wiring system instead of a standard RV power pedestal with the neutral-ground bonded inside the RV power pedestal, there may be a neutral voltage built up on the trailer's neutral buss due to the long distance to the neutral - ground bond at your house incoming power service.