My son has an older mobile home and has to replace the ceiling in the bathroom. There is no air space above the ceiling, but will plug with pink insulation. We are taking the fan down and putting it out through the wall. What I was wondering is what should be put up in the ceiling as ceiling board, any suggestions?

Older MH's used a composite fiberboard held up with ornamental screws. I'm sure the bath is fairly small, how about 3/8" drywall? What is the spacing on the roof joists?

I am not sure, what should they be?

Look at the screw that hold up the ceiling or use a stud finder. Most likely spaced 16 or 24" OC.

There are no screws that hold the ceiling up, but I think you are right.