[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