Four inches sounds like a lot of deflection - it must have a major affect on
the ceiling inside. I went out and looked at mine which is the same vintage,
and it has maybe 1" in the middle measured from side to side. Continuous big
snow loads aren't typical of this area.
Having removed a window today to recaulk it, I find the construction is the
fibreglass shell, rigid foam insulation and 1/8" wood paneling inside. The
roof appears to be the same. It looks as though the roof is supported by the
ceiling cupboards on one side, and the room divider thing beside the door on the
To reach the shell from the inside to try to reglue the foam, would appear to
require taking the whole trailer apart. If you did this, maybe you could prop
up the shell inside and glass one or more supporting ribs on the inside. I
suppose you could do the same on the outside if appearance is not a big concern.
One recent thread on this site has a smaller trailer, where the roof was revived
with the assistance of an ingenious roof-rack.
The amount of work I would envisage to correct this problem would have me looking
at other trailers, but lots of the people on this site don't quail at completely
stripping and restoring their trailers. So if you are handy and enthusiastic, and
this is a deal you can't pass up, you will be in good company.