Fill void with expanding foam?

OP,

Use low expansion foam. That said, it will most likely not provide any benefit and certainly will not help with insulation in a very small area. If your intention is to add the foam as a suitable base for the repair... consider just using this stuff: Solarez for the fiberglass repair.

I have used Solarez on my fiberglass sided trailer, as well as surfboard repair. Stuff works great, is long lasting and can fill voids. You do not need to mix resin/catalyst as this product uses the sun for activation. It really is a great product and can be found locally in surf shops where you live. I keep a tube of it in my TT just in case.

I would use the non expanding foam.
Bill

expanding foam... not a good idea... most RV's are constructed using staples.. not screws. you could push in the inside walls.

Rushing to do any job.. rarely works out as planned.

I had my home insulated with NON-Expanding foam... more money. harder to locate.
100 plus year old home. old style Plaster.

I see several potential problems and little, if any, benefit to injecting foam into the hole.

I would concentrate on repairing the fiberglass to keep the weather out.

Opinions are going to differ but with me, I think adding anything is bad idea. Your not going to get hardly any "R" value.

They make foams that will expand to varying degrees. I would get one that doesn't expand too much so that it doesn't expand more than planned and puts a bulge in either the inside or outside wall.

Bill