Thanks for the information, guys.

I got a 25x100 sheet of plastic up on one side of the house and anchored down with 1x2s. Will finish tomorrow if the weather cooperates, although wind is predicted.

I was suspicious of the plastic and the chimney, so after I posted the question, I built up a good fire and then felt around the base of the chimney on the roof, and it was warm. It looks double-walled, but with everyone here saying "Don't do it", I didn't do it.

Just this very second, it occurred to me that when I get to the chimney, I can stop short of the chimney and then surround that whole area with some stray roofing shingle tabs that my neighbor had left over. I couldn't see anything cracked or rusted at the base of the chimney. When my brother got here, he said it may be leaking elsewhere on the roof and running along a crosspiece within the "attic" and pooling by the chimney.

Stupid mobile homes. Once they're built, they should be good forever... (yeah, right)

Thanks for the help!

Sue