dh,
We don't have a lot of RV dealers around here. As to the butyl and mastic thing....what is the difference?
Butyl tape is a generic term used by glaziers. The two aren't the same, but if you walk into any glass shop and ask for butyl tape, you'll get the putty stuff that you asked for in the first post.
In my trim application I'm using an oddball trim. It is not a standard "camper corner trim". It is a plastic extruded product that has a cross section that is real simular. Whatever sealant I use it will have to work well with sticking to plastic.
Depending on the plastic, (polyethelene for example) nothing sticks to it permanently. RTV silicone seems to work best.
You'll need to try a test sample to make sure that your extruded stuff will allow the tape to adhere.