You've got criteria that are almost mutually exclusive. A retracting keel takes up a fair bit of room inside the boat, yet you want to have an inboard diesel as well... Finding both is not very likely on a boat that is less than 30'. IIRC, the in-board diesel on the RK26 is an OPTION, not standard. The drawings of the boat don't even show a prop shaft setup...
You might want to ask yourself, what are your real reasons for wanting a retractable keel and a diesel inboard engine....on a boat that is supposed to be light enough to trailer.
Obviously, trailerabilty is a necessity, since that is the first requirement you put listed... and trailering a boat without a retractable keel is a non-starter... why do you want a trailerable boat?? What kind of sailing are you looking to do??
BTW, most trailerable boats can't afford the complexity or weight of an inboard diesel.
Is an inboard diesel really a necessity??? Why is it a necessity???
I would like to find out which boats are equiped for my idea of the best boat for towing. Here is the criteria. 25ft to 30ft, Retractable keel, diesel engine. Of course the other necessary things like a mast that will stow etc. The combination of retractable keel and inboard diesel is what I cannot seem to find. Is it hit and miss or can I look for a specific manufacturer and boat class.