Although a big proponent of Foam construction for campers , I wouldn’t do any permanent Large building construction depending on the foam alone for structure... There are limits as to how Big you can go with any material ... and while excellent for a camper ... it may not be as good for a garage ...

However it may be Good when making “Panels” that would be attached to a more traditional framework (2X4s, 4X4s, etc.)...
(SIPS) , a proven building technology combining wood and foam into interlocking panels would also work ... You can buy or make those ...
Mary , living on a large patch , I’d just sink some posts and build a more traditional shed for the truck ... JMHO FWIW
And getting back to the first subject discussed here..... If you sink some posts , you can give them some extra protection by first using the mixture on them ( Poly and Min Spts) and then coat the bottoms , where it goes in the ground, with the Black Nasty stuff we use here for trailer bottoms ... Used that on my shed and then poured concrete around the posts , and it’s held up just fine over the years ... Yo can always do like the Indians did , and dig a hole , put a big rock in the bottom and put the post on top of that , filling around it with dirt ... That works too ...