Front cap separating from side of motorhome.

I would guess that it is probably a 1/4" blind rivet(pop rivet). To get the length I have used a piece of wire with a small 90 degree bend on the end. I inserted it in to the hold and then hooked it on the inside lip. This would give me the thickness of the material to be held together. Once you find the material thickness you can purchase the aluminum rivet that is suitable to hold things together.