AC issue

Here's some info from Chris Bryant's business web page. It shows the Blower Motor out in the open (readily visible with the plastic shroud off) and the Condenser with Condenser Fan out there too. The Evaporator, Room Air Blower and Drain Pan are all inside a sheet metal housing. This is pretty much the pic Chris had in the Blog post I linked above. . Left side of pic is front of coach and the RED Return Air arrows are the opening down into the coach where he warns about flooding with wash water, using towels, etc. I was able to use a deflector sheet (expired license plate) and divert spray from a garden hose nozzle away from that opening. It fills the Drain Pan quickly, though, so it's Spray Some, Wait Some. A pressure garden sprayer works pretty well and doesn't fill the drain pan so fast.
I really hope you'll get into this and clean those coils! And the Room Air Blower too if you can get at it. You should find one fin of the squirrel cage blower that lets you reach its Hex Head Allen Screw with a long enough wrench. I'm pretty sure the wrench is NOT metric, and either 1/8" or 3-1/16" size. I was glad to get ours off the motor and clean it on the ground, but you CAN work it on the roof, limited by the reach of the motor wiring. Bring a mat or plywood scrap as a work surface. Other than whatever it takes to remove the shroud, most of the rest of the work takes a 5-1/16" nut driver.

I don't think 55* is so bad. The statistic to follow is "Split," the difference between Return Air (from coach interior INTO A/C) and Discharge Air (cooled air FROM A/C back into coach). If it's hot in the coach and you're trying to pull it down, 20* Split is good. You may get even less.
Humidity's a huge factor. A lot of the A/C's resource goes into wringing water out of the air, so you'll get more split and faster cooling in dry CA than you will in humid FL.
Small HVAC units like Home Fridges, Home Freezers, Room A/C and RV A/C (nothing but a rearranged Room unit!) are SEALED Systems. The factory charges the unit to its design spec, pinches the tubing off, cuts and solders it sealed.
If you find a Service Port on one of the above, somebody's installed one to try to troubleshoot/repair it. If it's a "Bullet Valve" it's a sure sign of amateur tinkering. The valve itself is a Leaker.
An older RV A/C will be charged with R-22. Not impossible to get but not as easy as R-12 "drop-in" at the auto supply.
Understand this: These small systems (again, ALL of the above) are VERY sensitive to charge level. Go over and you're as bad off as under.
This is a better Tech Issues topic than Class C because all we RVers share these A/C systems. It'd really help to have Year, Make, Model of the A/C unit, and let us know if it's Ducted or Ductless. A Ductless can achieve better Split than Ducted. Those Ducts are up in a hot roof.
And if it's Ducted, the Freeze Sensor is a prime suspect. If it's not in the right place, it'll think the unit is frosted up and shut the compressor down.
Or DuctLESS A/C, Coleman Mach 15000, will shut down on compressor thermal overload if we run the fan on LOW speed on a hot humid day. The Evaporator Coil gets frosted.
You mention the thermal overload. I pray that is NOT your problem. At least in our unit, there is NO external thermal. The compressor incorporates only an internal thermal, so if it fails the compressor is junk.
You said "evaporator" is clean. What about CONDENSER? You have to get up on the roof, remove the shroud, remove some sheet metal, and look. Our friend Chris Bryant did a write-up on cleaning called Tech- Tune up your rooftop A/C that talks about it and used to have pictures. Now it seems to be only text.
I think you need to clean the CONDENSER area. If it's Ducted, you'll find the Freeze Sensor.
I cleaned an older Coleman Mach 13500 that I thought needed to be replaced, and once I got the gunk out of the coils and blower wheel, it served well for several years. Sold the coach with it still running at 25 years old. NOTE: Blower Wheel wasn't in Chris' blog post. I was into ours with blower motor bearing trouble and was appalled at the buildup of sticky grime in the squirrel cage blades. Pulling the blower requires pulling the motor from topside and may not be something you want to take on.
Let us know what you've got, what you want to do, and we can try to find pix or threads about it.

Most units are designed for user to add or qualified AC person. If you add a can you most likely will be up and running. These units tend to leak more in the off season when the compressor hasn't self lubricated it's shaft seal in months. Some units have been fine after adding and running them frequently.
If it leaks out fairly soon, then you have spent $5.00 to find out that you may need the $75 an hour mechanic. Do the logical method first unless you have $$ to burn. If yours leaked it all, it wouldn't be running at all now.
Brian

Cobra21 wrote:
You need to add Freon.

NOT. Most A/C units of similar design and use are NOT designed so that a user can add Freon. If it is low, "you" need to first find the leak and fix it.

You need to add Freon. When it gets lower, it won't run at all. It's designed this way to prevent you from burning out the compressor.

55° is 10° warmer than the air from mine.

Fish mojo wrote:
I'm sure it is the thermal overload relay for the compressor.

Question is, what is kicking it off?

Seems like you answered the question before you asked it.

If your conclusion is correct, then either the refrigerant is low due to a leak or the compressor is going bad.

It's probably the evaporator freeze control system. Just like most car A/C systems the compressor cycles off and on to prevent evaporator freeze up.