1981 Honey on Chevy

I guess it depends on several variables. I would think that a reasonable window unit would cool, eventually, unless it's 100 degrees out in direct sunlight. In our 28 class A the roof AC will cool it down nice. If we're at the beach in the direct sunlight on a hot day it will stay cool in the RV. But, it's 15,000 BTUs. On a hot day I would doubt that a 5,000BTU AC could keep up. In the grooming van we have a nearly new 13,500BTU and on a hot day with the washing and running the dryer it will really struggle to keep us cool in a 19 foot van.

Agree with above about A/C. Chevy chassis is good, in that size RV.

Repeating the common advice:

MAKE SURE IT DOES NOT LEAK (and/or that son can repair and seal it up)

It'll probably need TIRES regardless of Tread Remaining

Inspect, Flush, Bleed Brake System - Work likely needed there

RV A/C units are not usually small, they are usually the biggest that can be operated on a 120V 20 Amp circuit. Much over 28-30 feet, they install two.

There is a reason for this. With sun loads, air leaks, other heat leaks, and generally poor insulation, it takes a lot to cool a RV box, just as it takes a lot to heat it in the winter. My 29-foot motorhome needs a bigger furnace than my 1200 square-foot, built in the 1930s, masonry house. That's just for handling temperatures approaching freezing, doesn't begin to cover a couple of months of lows -10 F, which determined sizing of the house unit.

RV roof units start at 11,000 BTU/hr and go up to 15,000. Small window units are often around 5000 to 6000, I've seen them as small as 2,500. They put a 13,500 on a tent camper. Chances are the window unit is way undersize for the heat load. If in Missouri, you are where summer heat loads are high, as it is sometimes necessary to condense the water out of the air before cooling it beyond the dew point, and that is 40x the load of dropping temperature 1 degree Farenheit.

A 23 foot RV is going to have the same space as at least two bedrooms in a house. It also, as noted above, is going to be poorly insulated, relative to the average house. This will require at least a 13,000 btu AC, and my guess is that he put in a 5,000 or 6,000 btu unit.

I, too, would not mess around with a window AC, but go straight to an roof mounted RV unit. They usually come in 13,500 btu or 15,000 btu sizes, and are mounted in the middle for efficiency sake. They will be only a little more expensive than a similar sized window unit, but will be far more satisfactory, not least of all because they are built to take the bouncing around the camper takes going down the road, unlike the window units.

Many years ago my dad converted a step van into a RV. It was about 20'.
He installed a 5k btu air conditioner in the rear wall. It did a good job.
Maybe your SIL needs a slightly larger a/c or, a fan to help circulate the cool air.
In my opinion, the newer window a/c units don't cool as,well as the old units.

I wouldn't expect a window A/C unit in the rear window to cool the front part - there's nothing to drive the cool air all the way up front. This is why RV roof top A/C units are located more towards the middle. Maybe a fan would help move the cool air further forward?

RVs are not insulated all that well and a 1981 may not have much at all in the roof. With all of the sun beating down on the roof, one small window A/C isn't going to make it. Yes, he should install a roof unit and it should be a larger one if they want to be comfortable.

Bill