Well, I have re-used two of the backers, mainly because of limited space near the wheel wells. The others I have replaced at least for now with some washers from Brofasco and a buildup washer as the tech at Carl Cox RV advised. The washer is at least as thick as the original plate and covers the same area. I did keep the little squares and may in fact have someone cut some duplicates if I run accross a suitable piece at a scrap dealer.
I do have a theory however that the square plates are a different metal than the original carraige bolts which may have created a mild galvanic process, given moisture that would wick up the threads. That might explain why two of my bolts were pretty well in tact except right at the plate where they rusted right through. Then again, they probably didn't expect that old farts like me would be keeping these things on the road after 40 years!
Just for interest, galvanic action would certainly explaine why the original plain steel screws that held the aluminum windows
to the wooden frame dissintegrated. The worst galvanic corrosion occures when a small steel piece is used to secure an aluminum large piece. I replaced these with stainless steel - still dissimilar but tougher - and coated them.