My personal feelings is that I would not use anything other than the manufacture's recommended procedure. Since Ultra Leather is a synthetic, better and stronger than leather, and it cleans with soap and water, that's what I'd use.
For real stubborn stains, 409 or fantastic. 303 protectorant is used for seals, slides, etc., so what does it have in it for Ultra Leather? I'm just skeptical because I don't know the properties in 303.
Wayne MSGT USMC (Ret) & Earlene (CinCHouse) RVM14
Lexi - Goldendoodle
2015 Winnebago Tour 42QD
It is what it is, and then it is what you make of it.