Hi, I assume the RV owner has no manual for his Carrier unit or you would not be asking for assistance. You did not mention a model number for the Carrier unit but perhaps the attached manual link will match. The only reference to trouble codes is on page 2-4 but I did not see 7 flashes listed (did not read in much detail though).....only up to 6 so perhaps this manual is of little or no value for you.
I am not actually familiar with this carrier unit at all but it is strange the manual does not discuss the error codes. Maybe you saw the 6 but counted 7? After a quick internet browse I found someone talking about a label showing the codes is usually mounted inside the unit; access by removing the inside grill so at least you donít have to go on the roof. Not sure if this applies in your case or not. I also stumbled on someone advising in a 2009 post that Dometic makes (or made?) a replacement for the roof portion of the Carrier that allowed the ceiling unit inside to remain.....while just replacing the roof portion. Not sure if this is still possible but might be worth checking out if you cant figure out the codes or if an air conditioning tech cant fix it for you.
A newer posting reference to roof portion replacement of old Carrier V.....but using Coleman not Dometic. Still sounds expensive, but if you like your coach for many other reasons then its probably worth it.