I've done the same in the past with my rib I think the yellowing was some trees not too far away, I also had black dots, & heavy staining, all over my RIB.
I went to a local haulage depot, & asked them how they clean their trucks and the curtains on their tautliner trailers, as they seem to be made with the same sort of stuff that rib tubes are made from, and they use stuff called T.F.R. Traffic Film Remover.
The chap gave me a bit, and it's "BLIMMIN BRILL STUFFl" he told where to buy it locally, & I use it now & then, to clean the hard to shift crap of me RIB,
but after that, I always put on some inflatable& fender cleaner, from towsure, & me tubes shine like new.
Jif, or what ever they call it now, is quite good too but not as good as TFR.