Phil, we painted our l980, 3l footer, on the  inside with Zinsser "Bull's Eye" 1-2-3 Primer Sealer (white). It looked so good we did not put on a finish cost, It brightened the interior so that everyone coming in commented on how much more light we had that they. In retrospect, I would have, and probably will this summer, put on a finish coat. What kind? I have no idea what I should use but will
ask "the man." This is the point to get a color if it is desired.

MAIN FACTOR: Get the ceiling and walls squeaky clean. We used "Greased Lightning" (comparable to "Simple Green"--some of the others like 409 play havoc with my breathing). We
used it full strength from the spray bottle, small area  at a time (3x5, 4x6), rinsing it with clean
water as soon as the crud started to run. Went all the way through the trailer, and  then did it again. WORK!!! But then very important, we used a final white vinegar and water rinse, rather strong
solution. Harriett says "every little place; if you do not get it squeaky clean it (the paint) will not dry." And she reminds me that we did use a second coat of the Zinsser Primer-Sealer.

Exception was the shower stall. For some reason (maybe we ran out) we used a "Bathroom White" which I think is made by someone else. Afterward I read on that  label that it is not intended for shower stalls!!! It has done quite well, but it took 3 days for it to dry hard. (Too much humidity? Did not get all of the soap scum off? Wrong paint?)

Hope this helps. If you find some better way, let me know. BTW, Betty Fowler, at Sympsonia (sp?) Kentucky, the Land  Between the Lakes area, has a tried and true procedure for painting and for papering. And our own Sharon Weston is the resident expert on interiors!

Donald