I had something similar in the past but wasn't on a monty.
The fuel hard line had scaly rust inside that would collapse over to obstruct flow but would not blow out rust at either end of the hard line with air pressure. Tank and filter sock were both fine. Had power early in the drive but slowly tapered off, soon over 1/2 throttle = no more power , then 1/3 and 1/4 throttle horrible mpg no power and white plugs.
I finally isolated it to the fuel line by using my snapper mower tank fed straight to the carb. If you run out of things to test its worth a try .