APU and AC trouble

I have a pro-heat 1st gen? apu. Since I bought the truck (8 years ago) I have been unable to fix the ac which does not work. (everything else works including heat). I tore apart the bunk yesterday and evacuated and recharged the AC. The low side PSI is reading a negative vacuume with the compressor running, high side is around 100 psi (80F outside). The evaporator bulbs soldered onto and going into the evaporator core are both frosted. Putting a heat gun on them made no changes. Everything I am reading is that the TVX valve may be stuck, but I am absolutely unable to find anything that remotely looks like a tvx valve, I did find the filter drier which has the capillary copper tubes, coming from the sensing bulbs and going into it. I am not sure what to do next, i've tried tapping on everything, heating up parts, but no difference in colder air temp. Everything including the evaporator fan and the condesor(?) fan outside of the truck appears to be working properly... I hate quitting on DIY projects but I am totally stumped. I dont want to chop open the system and replace the drier when the problem is likely something simple or totally unrelated. *ANY help or advice would be appreciated.

 

also reading that despite the compressor starting/running that a bad or weak capacitor could cause a negative vaccum

 

put in 2 cans, low side should never be in a vacuum. Also frosting leading out of the evaporator core is likely a sign of either a obstruction (Drier/Filter? Capillary Tubes? TVX?) or maybe weak capacitor.

 

Both high and low sides do equalize when the compressor is off. Was around 90 psi I think.

 

No expansion valve was found anywhere. I know what they look like and I've follow all the lines. The frost is on the copper bulbs going into the evaporator. I've tried looking for parts diagrams to find the expansion valve, but no luck. Seriously traced all the lines. Read somewhere that the capillary lines some act as metering devices rather than having an expansion valve. Bought the truck with this ac problem and just trying to fix it now 8 years later... No clue in prior history

 

The frosting the copper lines going into the evaporator after adding a second can should indicate a problem should it not? After two cans shouldn't I have a low but positive reading in the low side, not a negative vacuum?