Darin,
I've used Por 15 on my frame and I also painted a trailor with it. I use the trailor out in the woods to haul wood, in fact the trailor sits out all year round uncovered and it's doing great. NO rust.

I also sand blast everything before I paint it.

Like you, I'm going to paint the inside of the roof and I'm going to use Por15. I'm also going to paint the underside of the body with it. I am going to top coat it with a different color, however.

So far so good is all I can tell you.