PCM?

I'm not sure I know what a PCM is....unless you mean the ECM.

Powertrain Control Module

How did he diagnose it?

The only time I've seen that happen on a GM truck, the PCM was fine. But it kept blowing fuses. It turned out to be a short in one of the oxygen sensor harnesses, which melted from the hot exhaust (pass side). This O2 sensors and crank sensor were the only components on the same circuit as the PCM. I had to isolate and test each branch circuit until I narrowed it down.

What engine do you have? Eddie Elk.

I would look for something simple before you replace the computer. Three times in 10 years I lost power to my dash, and ignition. The first time was a bad solenoid, the second time was corrosion at the battery disconnect and the 3rd time was a shorted out wire causing a 15 amp fuse to blow until I found it. Ask him to explain it how he determined it was the computer and post it here. They can fail but it is not common.

If the diagnostics are correct, go online & search for a PCM, new or rebuilt. You'll find them for a whole lot less. Simple disconnect & install.

All depends on the engine. PCM's cause a wide range of problems and symptoms. Some have their own computer and codes.

Try going to irv2 forum for your coach but pcm bad is unlikly

A bad PCM is very unusual. Unfortunately it is difficult for you to take it to get a second opinion. You could have a frayed wire or a loose connection which would give the same indications and be a lot cheaper to fix.
Some mechanics just automatically go for the most expensive part while they try to figure what is actually wrong. If that doesn't fix it they will hit you up for more parts until they really find the problem.