We use the in-line Camco blue-tube type. They're just cylinders filled with charcoal granules. We change the filter at the end of the season, no matter how much it's been used. We haven't had a problem yet.
I did use one recently, after cutting one end off and emptying out the charcoal, as a temprorary fuel supply for the generator. I had a short delay in my usual winter maintenance routine of running the genny every 4 or 5 weeks, due to time out for an angioplasty. When I finally got back to it, it isn't running. It will light off, run for about 5 seconds, then die. Maybe varnish in the carb jets?
More work needed - after overseas visitors are gone mid Sept. We don't use the genny very often and I don't think the original owners did either. At almost 9 years old, it only has 49 hours on the clock.
Frank and Eileen
No longer RVers or FR owners