sealing openings and equipment in outer walls.

For what you are doing I would just use the Butyl tape and trim off the excess once you put the screws in. It will not be exposed to UV light so I would not worry about that. Butyl rubber also comes as a caulk. When I worked at Sikorsky that is what we used to seal the airframe structure to prevent water leakage. It also has some adhesive properties but is primarily used as a sealer.

I don't recall anybody doing long-term testing, but when silicone is on black list, lot of other caulks can be good for boat applications but will not last long with UV exposure.
I think Dicor is safest stuff and easy available.
For interior marine caulks can give you better strength.
I never liked butyl tape. It is heavy and not a good sealer.
I think manufacturers like it because of clear appearance, but you don't hear about so many leak problem on RV without a good reason.

Lexel (Sashco) clear adhesive caulk. It behaves more like a glue, can be tricky to work with, very stingy and skins fast. Adhesion is excellent.

There is also Geocel similar to Dicor. Don't remember the name for vertical surfaces, I used it on the roof.

Bobbo wrote:
Don't use silicone. Ever. If you ever have to work on it again, nothing will stick where you used silicone, even more silicone. Get a good exterior non-silicone sealant.

Dupont 4200 is a very good choice. Be careful though, the Dupont 5200 is permanent. Really, really permanent.

There is also Dicor. It comes in two flavors, self-leveling and non-self-leveling. Self-leveling is good for horizontal surfaces, like on top of roofs. The non-self-leveling is good for vertical surfaces, like walls.

BTW, the "grey putty tape" is called butyl tape.

X2

Don't use silicone. Ever. If you ever have to work on it again, nothing will stick where you used silicone, even more silicone. Get a good exterior non-silicone sealant.

Dupont 4200 is a very good choice. Be careful though, the Dupont 5200 is permanent. Really, really permanent.

There is also Dicor. It comes in two flavors, self-leveling and non-self-leveling. Self-leveling is good for horizontal surfaces, like on top of roofs. The non-self-leveling is good for vertical surfaces, like walls.

BTW, the "grey putty tape" is called butyl tape.