i used expanding foam in a can...great stuff its called....there are different strengths or rate of expansion...
i highly recommend minimally expanding foam version and i cannot stress enough that you have to be careful with it....
i used it on the wall "studs" and nooks and crannies in my chevy express 2500 and it expanded so much (despite my extensive knowledge of the stuff and what it does) that it pushed the sheet metal away from the stud and i could see the outline of the studs on the outside sheet metal of the van...
i almost #(*$&* myself...it eventually subsided...mostly...i can still just barely see an outline...
so..aside from that...the stuff works incredibly well if you mind the above plus, dont let it get into drain holes or create wells where moisture collects...you also have to be ok with it being permanent...
end result for me is not one bit of condensation inside van no matter what the weather is doing or if the temperature swings rapidly....
i should mention that i boondock 100% in my van...i also have a fantastic fan that runs 24/7 pretty much
oh and you will use more cans of that stuff than you think