My guess is 20 gauge for my carpenter bus. It's definitely thinner than 18, that's for sure.
I didn't do it for all the windows, but I sheeted my drivers side window with our sheeting. You can see my documentation of it happenening here
I haven't driven the bus enough to tell you if I have any regrets, but I can tell you that I probably wouldn't use it as primary window skins on my next bus, but I'd be open to patching small spots like I did this one. It's a little too thin, turns out I like it thick. Also I don't like the screw holes it has.
Good on you for reusing your fasteners. I told myself I'd reuse mine, but couldn't find the heart to drive 30 year old rusty screws back into my bus.