Freon capacity with rear air 2004 158" Cargo Van

I have a 2004 158" cargo Van with auxiliary rear air. It is an aftermarket unit. It runs off the front air compressor. It does not have dual compressors. Can anyone tell me or venture a guess what the refrigerant capacity is? I bought the truck used, so I have no information on the unit.

This can be calculated!
However before moving forward try to see if there is a label with the weight somewhere on the unit or on a sticker under the hood.

Is there any I/D labels on the aftermarket equipment to identify its origin.
I have info on a lot of add on systems including Maxaire out of Texas which is a popular addition.

The more info the better, and it might help if you can see if the condenser/fan arrangement is slung under the van aft of the transmission. If so its most likely a Maxaire!
Dennis