Ford E450 '01 Dutchmen, power levels off on grades

Having a strange issue where I lose power on grades. It feels very much like there's a governor that kicks in... it just won't rev, won't drop into "passing gear" and even if I shift manually, it doesn't give me more. So far, it's gotten through the hill or grade, but something's definitely wrong. So far, it caps out at 50-55 mph on mild hills & grades and 30mph on more steep ones. Not that those speeds are wildly out of the norm, but there's no change in the pedal if I push for more... just nothing.

I found a post with similar issues and it turned out to be check valves on fuel line before the head. That was on a diesel forum though - this is the gas V10. No idea if those check valves even exist on the gas engine. Any insight would be much appreciated. I'm actually home in TX and the RV is with my father in Rapid City, SD. He's traveling to Portland, OR - so plenty of grades and passes. Hoping to have some direction before we call a mech. in.

Thanks!

rog