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 .

...on the TBI pics, I won't be able to get those until late April, unless I find a free moment. I'm off to New Mexico for work and have to ready the family, cars, and ranch things I'm in charge of.

Just pull off the intake hose so you can see behind the throttle body better. Its the smaller coolant hoses back there going into the tb. Those were a bit of fun to change out.

As for a update, so far it seems fixed, just thought I would start with the basics, i did another fuel filter, but the other one did seem to have no restriction.

Then i put about 1/2 litre of methyl hydrate in the tank, and i must have had water cause as soon as i added it, seemed like the problem was gone!!!!

Turns out my fuel pick up where it bolts to the tank leaks gas so maybe some water got in there

Looks like another project!!!