I know this is bad to say but you need to find another A/C man. No one is going to be able to fix this over the internet without seeing what is actually happening. I would need to see what the manifolds are reading and watch what they are doing.
Couple questions for you:
Has the system ever been opened?
Has the compressor been replaced?
Is the condenser clean?
There could be a number of things that could be causing this but your voltage is fine.
It could be as simple as the $2.00 orifice. You could have a blend door problem in the dash. Could have too much freon. It could have too little freon and freezing up. It could be a weak compressor. Compressors can work fine at low RPM and then not work at higher RPM.
Like I said, without having the manifold gauges hooked up in front of me it could be a number of things. I had a Dodge dealer warranty a compressor 4 times in two months and it ended up being the orifice tube. I had to do the work myself to get it right. PM me if you want to talk about what's going on with your truck (what's been done and what they are telling you) and we can seen if we can get you cooling. A/C systems are not hard to work on it's just everything has to work properly or it won't work at all. The hardest part is figuring out what's not doing it's part. ;)