the foam can have drawbacks yes, your right about it drawing moisture, thats how it reacts and sets, so you dont want to be doing it in damp or wet sections, once it has set it will be like the lagging around your hot water tank, if the surface is broken or damaged then yes it will absorb water, but as your putting it in a sealed or protected area then that shouldnt be a problem, shrinkage, hard to tell, but i have only ever seen it shrink when exposed to sunlight, and as its a cavity filler, then that shouldnt be a problem.
how well it works can be down to a number of things, like previous condition, how much you use, whether you get any air trapped in there etc etc,
Stuff to recommend, well the fire retardant stuff doesnt seem to expand quite as much and sets slightly differently, but i guess that would be the stuff to try, any repair done near it is going to be hard later though, but you can get a solvent that dissolves it,
sorry to hear the waxoyl didn't work, and yeah i forgot you had a different van before






