This has already been covered but I thought I could add to the issue.
My 2002 U320 has the PowerTech 10KW generator with the Kabota engine. Model: V1505-EBG1 per the manual.
Apparently, it is hit or miss on getting an alternative secondary fuel filter to fit on the engine. The Kabota filter 15221-43170 fits fine.
I'm guessing that on the engine filter mount the threaded tube must be supplied or installed at different lengths. On my coach is barely long enough for the Kabota filter to engage some threads. The alternative NAPA 3390 listed in the Foretravel manual is recessed in the center and needs a longer threaded tube to attach. It's a little hard to see in the photo but the NAPA filter has the center recessed more than the Kabota. Some people have said that the NAPA filter attaches. Others have said that another NAPA filter would attach. I've seen the Donaldson P550127 attach.
It seems strange that the threaded tube is so short. The mounting looks custom so there must be some variation in the build. It looks like there are alternatives that will mate up with the short tube.
The other thing I figured out was that it didn't mater what your fuel level was, as long as, you use pinch off the fuel line.
https://www.amazon.com/dp/B08H85NN64?ref=ppx_yo2ov_dt_b_fed_asin_title
I wonder what different aftermarket fuel filters can attach to the shorter tube mount?
I run the Fleetguard FF5226 there. It has a 20 micron rating. The Kabota filter has a 24 micron rating. Looking up the Napa, it has a 10 micron rating.
My coach also has the fuel/water separator outside the gen box that runs the FS19802 filter, which also has a 10 micron rating. I run Fleetguard filters in all positions on my coach. Good enough for Cummins, good enough for me. :)
Andy, is it possible that the threaded tube is adjustable? Just a thought.
I have no idea what I was working on but I had the same problem that I could not get the filter to grab. I then grabbed the threaded nipple and was able to unscrew it a bit to get the filter to grab hold.
I did the same as turbojack and screwed the pipe out enough and then used lock tite on the threads and now any of those filters work.
I did wonder if the tube might unscrew more, but surly it was designed that way!?!? I'll look at that when the fuel gets low again. This may just be a factory assembly issue. Someone didn't Murphy proof the assembly design.
(Grumblings of an engineer)
My generator came with an. FF 42003 so that's what I replace it with.