Welcome to the forum!
I'm a bit surprised that no Redcat owners (which I'm not) have stepped up to answer this yet. I'd have to guess, but in general terms it's not hard to see what's going on. Your 2" diagonal has 3 issues, compared your old diagonal, that all contribute at least to some extent to lack of focus: (1) it's bigger overall, so the optical path in the right-angle section is longer, (2) the EP holder is also probably longer, to accommodate 2" nosepieces that are typically longer than on 1.25" EPs, and (3) it's mostly air inside, as opposed to the prism which is mostly glass. (Converging rays come to focus over a shorter distance in air than they do in glass, so in other words, you have less room to play with, with a mirror diagonal.) And by the way, if you're using a 1.25" adapter with the 2" diagonal, the thickness of the adapter has to be considered as well. In short, your optical path is just too long. If you can see an easy way to shorten it, give it a try!
Otherwise, if you can live with a 1.25" diagonal, it should be possible to find an adapter with the proper thread size to make your old one fit.