[Tf175]

Hello Gurus , hope you are staying cool . I am hoping someone can shed light on my air conditioning adventure .
Our '86 has 3 roof air units , Dometic Duo Therm 15k , appear to be original. 
The front unit works well , ran almost continuously on a trip last month. 
The mid and rear units will freeze up after about 1.5 hrs in 90 degree + outside temp.
I have cleaned the coils , they are not as new , but you can see through the fins .
Current draw , according to analog ammeters above driver : Front ac unit : 14 to 16 amps with compressor running .
Mid and rear ac units : 10 to 12 amps .
Fan motors all turn freely , seem to move plenty of air .
I looked at mid unit this morning , there are two service ports which appear to be original equipment . Information on compressor indicates the system uses R 22 . 
I have read somewhere , either here or elsewhere online , that a lower current draw is an indicator of a low charge situation . 
I don't have a clamp on amp reading meter , but they are available and I can get one for a more accurate way of measuring .
 Any input greatly appreciated !
Thanks , Chris

[Richard]

Low current draw and freezing coils are classic tell tales of low charge. Those units typically do not have ports for measuring gas pressures. Almost all RV techs consider them to be disposable units. If you have a refrigeration buddy, ports can be added and the units recharged. However given the labor cost, the age of the units, and the exorbitant cost of R22 today, you may be better off with replacing the unit.

[Tf175]

Thanks Richard ! As stated in my overly wordy post , the mid AC has service ports , not yet sure about rear unit. I will speak with the guy who services the heat pump at our shop .
Thanks,
Chris