I used 4x8 1/8 inch plywood to refinish the ceiling in my fiver. One coat of KILZ primer with a coat of semi gloss bathroom paint applied with a high nap roller gave a close approximation of the finish in the rest of the trailer. In my case I could not find the vinyl coated plywood in any place other than a dealership, which wanted to charge me $75 a sheet.
I didn't use H trim, Jayco didn't either. I used batten trim, prefinished in oak. stapled over the seams. It looks great.