The place sounds really cool. You should post some pictures.

You might be able to insulate the top of the roof if you use metal. Put sheets of insulation down and screw the metal panels through the insulation into the wood below. Again, that may not satisfy code. I know modified roofs are often put down over insulation sheets though. most of those are flat or close to it.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13