Cosmic Debris (boy, that has to have a tale behind it). I use the same method you use. Because I have slide awnings it makes it a little challenging to get protectant on all the seal. So I treat 3-4 times during the season. Perhaps I am missing something but I have never treated the interior of the rubber. The exterior rubber receives the most abuse with sun & elements so that's all I have ever treated.
I treat with Thetford slide out rubber protectant.
Since my rubber seals sometimes don't fold all the way out when the slide comes out, I use a yard stick (edges sanded to a nice round) and slide that between the rubber and the slide. Works pretty well.
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