We had a fellow on another forum last month who tore up his roof and replaced. He said it was a two day job for two people, with the second person mostly a go-for and extra pair of hands rather than a skilled worker. His was a rubber roof, but the underlying structure is the same, with wood core sheathing. Your biggest problem, I think, is to find a covered area to work in, though I suppose a big tarp would do for weather emergenices and overnight protection.
You might consider putting EDPM rubber on instead of fiberglass. It's probably an easier solution for a do-it-yourself job. Once you replace the spongy wood, the rubbersheet is just glued down.
Here are some comments from the other fellow, who went by the name "txcampernrh".
Some tips that I learned:
1) Remove all the rotted wood, trying to use something questionable is not worth it.
2) Use a 3/8" plywood to replace deck, we used 1/4" but is a little to weak.
3) Make sure you use a good staple gun, not large nails etc, cover ALL staples with a good duct tape to keep from backing out during trips. This is very important according to all the experts I spoke with.
4) You must put wood putty in all knots, are holes in wood, let set up than DA sand entrie deck with 120 grit. Do not putty the cracks between boards, they need to move.
5) Once you get the EPDM rubber on the roof, roll out a few time to make sure it will cover and is going straight. The is time costly but is a must do. once you are sure it is correct, roll EPDM up so you have 2 rolls in the middle.
6) Cut 4 pices of a light paneling and cover the entire edges with duct tape, they should be 2' x4', these will be used for you knees and feet as you work the EPDM out to get it to stick. Place 2 on one side and 2 on the other, should cover the entire roof on most campers. Were just Sock feet....
7) Spread the glue on half the roof, you want a thick layer of glue, maek sure roof has been blown off good after sanding. Once glue is spread on half start in the middle and roll out a couple of feet at a time. Use a rolled up bath towel to work the EPDM toward the sides or any of the openings, vents,A/C. You have plenty of time before the glue sets up, so do this real good, no hurry.
8) Repeat on the other end of camper, once the material is laid down you can install the drip rails and end caps, make sure you install the crown moldings before the center trim pieces. You will use Butyl tape under all drip rails and molding. Wait at least 48-hours before installing vents, A/C, Antenna etc.
The job is time consuming but definitely possible for a newbie, the estimates I got were from $4500-$5700 for a 28-30ft camper. I have less than $1000 in materials and about 80 man hours total so far, going to install A/C today and vents.