Yeh, Lee is right!  A/Cs tend to last forever; and when probelms
arise, they are invariably inexpensively electrical in nature. If not
the start/run capacitors, the next suspect would be the compressor
relay and associated relay-delay circuitry.

ect.

If it's a capacitor the unit would draw a lot of current and which would
likely trip the breaker.

Could be a failed klicks-on (spl) or other overload cut out.

Testing requires a VOM and a bit of technical ability.  If you don't know
the drill you could get hurt.

'
OK Dave,

Now that we know something, let's try to get you along a little bit
farther.

Lets also try real hard to not get you killed while we are doing this.

The compressor doesn't seem to run (by the by, many get cold when they
run - unless there is something wrong).

Before you climb up on the roof, open the breaker for the AC and then
turn it on MAX COLD.

There is a box (so called) on the compressor.  Two wires go in. (Maybe
two come out to a capacitor.)
Push the spring that is over the top down to open it.
Look in that box.
There are two things in the box,
a temperature limit switch (round thing held on by a spring) and
a start relay that is usually plugged right onto the compressor
terminals.
One of the two inbound wires goes to the switch and one to the relay.
With the unit unpowered (circuit breaker open), pull a wire of the
temperature switch (Klikson is a brand name)(either one- it won't
matter) use the ohmmeter on the switch - should be Zero
If it has a capacitor, pop the little rubber cover off and remove one
wire.  Put your ohmmeter on it it should jump down and back up to
infinity pretty fast.  Now reverse the meter leads and do the same thing
again.  The same thing should happen.  If yes, the capacitor is probably
good.  If not unscrew it and find someone (like an electric motor shop
that can give you a second opinion.
Because this is a hard wired unit (most RV are) we can't do the next
test the way I would like to so......  (This why you turned it on.)
Now measure resistance between the two wires that feed the compressor.
Because you left the fan at max, this will read a small number of ohms
IF the thermostatic switch inside is working.
Last cold test.
Measure between the switch wire you removed and the other inbound wire.
This should be an even smaller number than you just got.  IF it is
higher than the fan, that is not good (it is also very rare).  It could
still be the start relay, but we are into really rare.

Only if nothing so far gives a definitive answer should you try the next
trick.  I would advise against doing it alone.

Reassemble any wires you disconnected, and power the unit up.
The fan can be on low.
Set you meter to read AC Volts scaled for 125.
Now carefully measure the voltage between the two feed wires.
I hope it says Zip. - Your problem is downstairs and cheap.
If it is close to line voltage, not good.
If you hear the compressor going:
CLICK---HuMMM-CLICK----------CLICK--HuMM-Click.
Shut it down, the click is the temperature limit switch which is also
kind of a circuit breaker protecting things.

Well, I'm tired and my brain is getting fuzzy.
If this doesn't get you there, come back with what you find and we can
try again.

Best of luck

Matt Colie