The seam tape on my '72 Boler
was more than 1/2 gone. There are replacements out there that some people have used. I personally just tore all all the seam tape, filled the gaps with white paintable caulk and moved on. I like the smoother finish anyway.
Your insulation material:
I assume it's the same foam-rubber-like material as on Bolers? If so - when you say it's in bad shape, what egg-zactly do you mean? If it's just a matter of the seams and a little (or a lot) of discolorization and dirt - I think that it would be best to clean it up and not try to replace it all.
My insulation was REALLY nasty with mildew and was also falling down in places. I glued it back up with construction adhesive (liquid nails), cleaned it well (Tilex bath and tile cleaner) and eventually repainted it all (Rustoleum's spray vinyl - it's sold for fixing up car seats and dashboards. I used it on my insulation (white). I also used the black spray vinyl on all the rubber that was looking chalky. Gave it that nice new luster. :)
For the insulation - between spraying the insulation WHITE and replacing a dark carpet with light
(mostly white) floor tiles - the inside of the Boler
was considerably brightened. Now it's just the defects in the internal fiberglass furniture (seats & dinette) that bugs me. :)