Maybe try por 15 seam sealer.   www.por15.com     I had the same rot on my
85. I went to a auto body supply shop and got a product called 3M rust
fighterI.Spray on and remains tacky.Seals out moisture. The westy driver
seam ,rusts from the inside , out. Bumper end caps , when hit in a
parkinglot, will crack the seam sealer. the rust will go fromthe outside,
in.That is the cause of the seams rusting on the corners of thevanagon.I
got pictures that look just like yours,with the camper stuff all torn
out.Check behind the drivers seat and passengers seat, where the floor
sheet metal and battery boxes meet. I had rust there also.While you got all
that stuff out, get a piece of sound proofing material from the floor of a
90 -93 carava($5).It willgo neatly from the engine , to the front seats,
under the wooden boards.