Leon... I had a similar problem with a 76 motor home we inherited from my father-in-law. It would run well on level ground, but wouldn't climb a hill. Ol' Stan used to complain about clogged fuel filters all the time, so when it acted up on a cross-country drive, I pulled over and tried to change it. There was a new spare in the glove box. I put the new one in the way the old one came out, and the engine ran well for a while, then acted up. As I was being jerked back and forth in the driver seat I thought about how the small, thimble-shaped was situated in its mount. The closed end was facing the gas line. I started to think that maybe that wasn't right. I stopped and flipped the filter around to see if things would improve. Well, the old 454/4bbl started pushing that shoebox-on-wheels uphill faster than I was ready for!
It's absolutely amazing how a trickle of fuel can fool you into thinking everything is alright at idle or low load, but will make you tear your hair out when trying to make speed!
Good luck

Current fleet (subject to change w/o notice)
\'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run.
My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787.

Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.