In answer to your original question.
3M spray on adhesive sticks well to ensolite. Available at the big box home improvement or hardware stores. There is more than one flavor some with different strength or properties.
I used the 3M Super 77 as a "helper" when I put my seam tape on. Hold spray nozzle close to seam, spray a line along the seam, let set for a few minutes, apply tape. Tape stayed put in 100 degree heat.
Can directions state that application of adhesive to both surfaces yields a stronger bond if that is desired. I just applied to ensolite to make removal/replacement of tape in the future easier.
Caulk over caulk is not usually a good idea, clean surface is generally required for good adhesion of anything. Probably a little less critical that it be totally clean since your not "sealing" anything just filling and blending a seam. But the more you can get it down to solid surface the better.
Just FYI the reasons I went with seam tape from Scamp
store over caulk (which is not to say caulking seams is a bad idea) I liked the fact I could easily replace if it became dirty or damaged. It was fast, I removed and replaced one end of camper in an hour with tape. And my 77 scamp windows
have no inner trim ring, tape is used to seal/hide the seam around window, a location I would not caulk as it would make future window removal very painful. Having to use tape there it just made sense to me to go with tape on the rest.