Hi Alice,
We are working on fixing up our recently purchased 1988
Perris Pacer, and one of the first things we did was replace the vent. We bought a Fantastic Fan. Just make sure you get the right model--they have a bunch of options that affects how much sticks out the top of the roof or the ceiling. (But the standard size of the vent matched the dimensions of the Fantastic Fan--which made installation pretty easy and straight-forward.)
It works great. We picked it based on reviews we read on this site.
Good luck!
Janine