Going by what I've been told by other caravan owners, they work very well, most of the smaller 2 berth vans use 1, 4 berth up tend to use 2, how well it worked in a garage I don't know, as most caravans are well sealed against moisture to start with, apart from of course the vents they need for drop outs(gasses etc).
I think most caravans are also double glazed, well mine is anyway, and that's not a posh one or anything, and it's 1992 model.
At £22 a pair even a couple of pairs won't be dear, and should certainly do the job.
One guy leaves 2 in a winnebego(sp) motorhome for 3 months at a time, and it is huge, got a 39" plasma tv, sleeps 6, even has it's own car towed on a frame, think it's a bit posher than mine, but the man is totally down to earth, he says one of the best things he ever bought, the other is the motorhome as when he comes home drunk he has to sleep in it as she locks the doors!!
tabetha