I think Gumpy means butly rubber tape, it would be eaiser to work with on the side of an Bus. It is available at RV places, make sure you get butyl rubber and not putty tape.
Work?/Play safely Jim
Actually, I was thinking caulking when I wrote that, but I guess I wouldn't necessarily pick one over the other. I've used both, but find the tape is harder to find. I've had very good luck with caulk, though it's very messy. My solution is a putty knife and a plastic grocery bag used as
a glove. Scrape the majority of the excess off with the knife, and clean it off in the plastic bag. Then, using mineral spirits and several clean rags, clean the residue around the edge and on the frame.
If you can find the butyl tape, that might be easier to work with, but does not fill voids and seams as well as the caulk (may not be an issue for you, it was for me).
I DO NOT recommend using polyurethane caulk. It does a great job of sealing, but if you ever need to take the windows out, it's nearly impossible to get them out. (Not completely impossible, but don't ask how I know that