That's some good advise about the drain holes in the rear panels Chris. If
you got A/C then the drain tubes from the evaporator pan empties there also
so in the summer it's probably wet all the time. If you drill holes thru the
metal, the raw metal holes need some kind of corrosion protection or they'll
be another rust hotspot. Best to put some touch-up paint around and inside
the holes.
Auto manufacturers use a compound called body cavity wax to seal inside
panels to protect against rust. Great stuff, it's semi fluid and runs down
into seams and tight areas between panels. The stuff never really hardens but
does stay in place. Most body shops use special wands with 360 degree nozzles
to apply it into frame rails, rocker panels and Quarter panel wheel wells.
Some of the body shop suppliers like Kent, Wurth & Crest sell spray bombs
(cans) of the stuff. If you know anyone in a shop a shop supplier talk them
into selling you some of it. It might help prevent or slow down some of your
rust.
Another rust zone in the campers is a body seam underneath the stove/sink
cabinet. Had mine out recently and noticed that the area near the sink drain
was rusting. Started pulling up the seam sealer and there was trail of rust
running all the way to the back of the floor. Took alot of work to clean it
up and not sure but it may come back again. To see if you have a problem open
the cabinet under the sink and lift up the floor panel the hose goes thru.
You should see the drain flange and a small portion of the seam (joint of two
metal panels). If the sealer looks loose any where, don't be shy; pull it up
and see what's under it. Remember, rust is CANCER and the sooner you fight it
the better.
Sorry about being so long winded, hope this helps someone's Van.

Tom Smith
83.5 Westie
Charlotte, NC