Loosing speed going uphill and revs at 3500 rpm+

Probably plugged fuel filter.

Mechanic time. Were your water, oil and tramission temps in normal range? You were checking them as you climbed, correct? Your MH isn’t a horse and doesn’t need “rest.”

Dirty fuel filter, simple solution.

Or the fuel pump is shot, hard solution need to drop the tank to replace the pump.

navegator

How steep is the hill? Could be normal. Check the air filter too.

If it was fuel filter, I don't think he'd be pulling 4000 rpms with his foot to the floor?? Clogged fuel filter would not let it rev that high as far as I know.

Probably normal, and he just does not expect that kind of performance loss going up the grades?

Not that it makes any difference, but what chassis is it? (Ford or Chevy).

Good luck!

Mitch

Check for plugged exhaust, especially cats if any.

Loss of power was the first thing we noticed when my daughters car had a plugged catalytic converter.

did you try downshifting?

We first need to know what chassis and engine? And where it was you were driving and what grade?

darsben1 wrote:
did you try downshifting?

He was doing 4000 RPM. Not sure down shifting is an option.

You may be maxed out on power but it doesn't hurt to check the things others have suggested. I fit has more than 50K miles on it, I would swap out the 02 sensors as well. They may work good enough for the computer to not turn on a light but they get lazy and rob power and fuel efficiency long before throwing codes.
For domestic products, they are usually very cheap and as easy to change as a spark plug.