Just as an FYI: roof top units don't operate quite the way the engine mounted/dash AC's do. While you can "freeze" your hand over one of a car/truck vents; the roof top unit is designed that it's output temperature is 17-20 degrees lower than the input temp.
So simply check the temp difference while you're up there cleaning the unit out.
This is because there's no outside hot air exchange that a dash unit could have. As the RV cools down the inlet temp also gets colder so you don't have to be afraid the AC will only drop 20 degrees from the outside temp.
As for life span, IIRC from the manufacturer's notes has a mean time between failures of 12 and 15 years. Proper maintenance/cleaning/ and storage raises it. We have units from the early 80's that are still running like clockwork, but we also have morons that ignore the units, let them clog up, etc and replace them every couple of years.
It is easier to get tired of them than wear on out. I have thought of replacing the gasket seals on our 1993 and swap front to rear since I think it has never been used. The filter looked new and was clean unlike the front one. The one in the bed room is too cool for sleeping. Just to same the front one in hot weather sometimes I just run dash and rear unit. We can only run one or the other at 30 Amps.
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