1998 Minnie Winnie ceiling A/C not cold

You can get a Coleman Mach Parts Diagram/List here. On the Coleman, only four fasteners hold the plastic cover. If it's an older unit with the original cover, handle with care. They get very fragile with age and you may find you need to add metal plates under the fasteners when you put it back.
Then you can see the compressor. If you have a clamp-on ammeter, you can see what the compressor current draw is when an assistant turns the A/C on. There's a rating plate on the compressor. "LRA" is Locked Rotor Amps which is what it draws if it doesn't start. It may not start if it's failed, stuck OR the Capacitor is bad. To see the Capacitor(s) you need to remove a wiring cover on the driver side. There should be two silvery-looking Run Capacitors. One for Fan and one for Compressor. It's common for the Compressor Capacitor to fail. If it looks Swollen, Bloated, or like Something has Leaked Out, it's shot.
Run Capacitor looks like this

May also have a Start Capacitor that looks like this

Meters to test don't have to be expensive. From Harbor Freight
Clamp On AC Ammmeter $13

Digital Volt Ohm Meter (DVOM) with Microfarad (Capacitance) Capability $20

Please post the Model Number of your Mach and I'll look at the parts list. It may show a Start "Device" in the parts list but not actually have one depending on the details of your exact unit. If it has both a Start and a Run Capacitor for the Compressor, BOTH have to be good. AND the Device that takes the Start Capacitor out of circuit once Compressor is running ALSO has to be good.
Bear in mind, the Compressor has an internal temperature safety down deep inside it. A cold compressor should test as I described. If you've been trying to run the system and compressor hasn't been able to start, it'll get hot and that safety will keep it from starting or drawing current till it cools, which could be an hour.
What do you think? Want to go after it? Let us know...

Thanks for the ideas. Great thoughts! It is not a ducted system - just blows from the unit. Not sure if the compressor is kicking on - I did get on roof but didn't pull of the plastic shell, just peeked in to look for corrosion or stray wires. Unit is a Coleman Mack, has two dials. One is for fan speed and fan vs AC. We have been running that on "hi cool", other is for temp control, have that fully clockwise on "cooler". Wall thermostat is "suburban" - have it "on" and slid all the way to coldest setting.

How would I know if the compressor is running? What would symptom of failed capacitor be? Have some handiness but problem is my own time... Probably have to use shop if beyond a quick fix?

What is the Make/Size/Model Number of the A/C?
Do you have ceiling ducts, or is this a non-ducted unit (where all air is returned to the coach right at the indoor part of the A/C)? Please describe your controls...
Wall Thermostat - or - Knobs on the A/C unit itself?
If it's a Coleman with a Knob, COLDEST is all the way Clockwise on the Knob. Is that the way you turned it?
RV A/C units DO fail from bad compressors and leaks in the sealed (Freon) system, but problems like Capacitors are much more common.
Can you carry out some troubleshooting instructions if we provided them? You would have to get on the roof, remove the plastic shroud, and remove a couple pieces of sheet metal to get at the components.
Do you have at least a test meter?
Meters measuring MicroFarads (capacitor test) and AC Amperage (compressor current draw) would also be helpful.
This is a helpful cleaning process. It'll explain how to get into the rooftop unit, and be a very good thing to do if we can get the cooling part running.

Is the compressor kicking on? I could just be a failed start cap or even a loose wire if it's not running.

The A/C on your roof is like a window unit, just a different shape. There are a few things like cleaning it, replacing starting capacitors or blower motors, some control boards, that can be done at reasonable cost, but the refrigeration unit is sealed and not meant to be serviced. Some shops that service RVs might be willing to tap a refrigerant line and add refrigerant, but what suggests that is underperformance reather than failure to cool. For getting into the core of the unit and trying to rebuild, shop time is often more expensive than a replacement, and it is hard to find a rebuilder.

RV dealer that does service can likely diagnose and figure out whether the problem is big or little. Expect to pay at least an hour of going shop rate to have it looked at.