Here's some things I discovered when bringing my AC back to life:
* -45F is about all you can expect at 90F OAT, but at 101F I got a -47F drop; and every -2F is meaningful.
* Don't try to service your AC in 75F weather or below.
* Dry air will give you better results than humid air.
* And it pays to clean your condenser back by the engine. Note: You can point an infrared temperature gun at the condenser and sort-of pinpoint temperature blockages if you system is not performing very well, but to clean the inside of the condenser is a PITA and expensive to replace.
* Most AC techs will tell you -40F is typical for RVs, but that's because they attempt to use the same type of AC service on a car as they do on an RV. So don't be satisfied with -40F. Your RV AC system can do better.
* There is a placard on your firewall (above the generator) that should specify from Freightliner how much 134a your AC system should use. In my case it's 40oz, but I recommend putting in +10% more or 44oz. The system can handle 10% more and maybe you will not need a recharge 2-3 year later as some 134a will leak out over time. So the extra 10% may buy you 1-2-3 years more service time.
* Sometimes your AC will start blowing cold when you overfill it, and add 2x the amount of 134a to the system, but you will never reach optimum cooling. I.e., when you lose track of how much 134a is in your system you have to start over and evacuate your lines... and then add 40oz... let the system stabilize for 15-30 minutes; and then after you can add the extra 10%.
* I cheap digital meat thermometer will work fine for determining the AC drop in temperature.
* If you need to evacuate your system, I recommend you replace your AC-Delco Air Drier-Receiver. It's only $30 and and the desiccant inside can block or restrict flow, which maybe the reason why your system is not blowing -45F colder than the OAT.
* The real tip is that you want to get the old "brown" PAG-100 oil out of your lines. So if work with an AC shop make sure they do this... using a high power vacuum device... and then you can replenish your PAG-100 oil using the PAG-100 cans you can get from Freightliner or Amazon that include the "ICE ADDITIVE". My guess is that you will need 4 or 5 3oz cans (12-15oz total), which is another $30 I would guess. (Depends on how much PAG oil you can get out of the system so try to reclaim it to know for sure, then add 3oz more.)
For more tips you can download the attached .pdf. Note: I just slapped this information together for reference, but it still should help you know what part numbers you need and a bunch of other things.
Here's a basic video for on how to evacuate you lines, for all of you who don't know how to service your AC. It's not rocket science, until you AC system does not want to work right. Then it sometime does take an experienced AC tech to know where to look.
The parts are relatively cheap, and if your 10-15+ year old RV is not blowing cold enough, but it is working, I.e., not leaking 134a, my guess is that you can do your own AC service: replace AC-Delco Receiver Dryer & Mercedes TxV for $60 and then buy 4-12oz cans of 134a (but only use 44oz) for $25; and $100 at Harbor Freight for the evacuation pump... just don't forget to order the adapter so you can tap into the 134a cans from Amazon.
Attachment: Dash AC Service PDF