parksmart aux A/C unit

Thinking about 8000 or 9000BTU, sure it may be OK but it depends on whether there is hot sun on the outside of the cab and how well insulated the cooled space is, and how air may leak around the sleeper curtain, and leak in/out of the cooled space, as well as how much heat the curtain material itself conducts from the front into the sleeping area. Lots of little things. Once the space is as cool s it is going to get, the inefficiencies and air leaks can be chased. Presuming the system is working to its rated specs of course.

No one has yet mentioned an infrared thermometer, a great tool for finding things that steal your cooling from you. Don't buy the cheapest one, they are always a few deg off at some points and correct at others. It's not linear. The type the HVAC guy uses is best and it aint from harbor Freight.

Then one can entertain oneself a bit by letting the sleeping space cool off, and start checking the walls, ceiling, floor, the curtain temperature, places where wiring or conduits pass through, etc.

Find the inefficient (warmer) spots and try to do something about them. The cold refrigerant lines could also be insulated. A warm conduit or hose can be insulated. The foam noodle type stuff they sell for home a/c lines works. If it is warm under the bunk where the machinery is despite the space being cool, where is that heat coming from? conducted from outsiide or maybe from a blower motor or compressor? Some can be done something about, some can't. The cab floor is a huge culprit in 4-wheelers cargo vans. How about tractors?

My point is that presuming the system is working properly, then it's time to look for other little issues that steal your 'cold' from you. If there's a 1" gap the width of the sleeper curtain across the top or botton, that is a huge leak. The high pressure line from the compressor to the condenser is always hot. Insulate the indoors part of it if possible. Enought small influxes of heat add up to a waste of cooling. They really add up. Something else to consider, if you are a polar bear like me, is that the manufacturer's idea of 'comfort' may not be the same as yours. The EPA has also fought against higher capacity a/c systems by mandating(?) the use of as little refrigerant as possible. The result is smaller condensers, evaporators, compressors, etc. So, every thing that wastes the cooling can found and potentially remedied. Those who are more handy could invest in a set of gauges for the refrigerant in use. It takes some knoweldge to understand how to use gauges, but youtube has some good people.

I'm not an expert on a/c but I fix my own central unit at home and I also fix my vehicles' a/c's. Using the infrared thermomenter, I found several little issues in an E-150 cargo van and fixed them as best I could. It made a large difference. The main one was obvious - insulating the cargo partition and closing the gaps between it and the ceiling, walls. YMMV