The ghosts of RV, flower, and dog shows past haunt the barren Bayside Exposition Center in Dorchester. The 275,000 square-foot trade show crypt might pass for the creepiest place in town if not for the company of Keith Motley, the eternally upbeat chancellor of UMass Boston. The university should play to its strengths when developing the site. Chancellor of UMass Boston. “Every time I go in here I get excited,’’ said Motley. “We can have it all right here.’’ When UMass Boston bought the property last year, college officials made noise about retrofitting some of the expo spaces for classroom use. Motley clings to that belief. But the university’s architecture and planning consultants want no part of it. They know that no part of the bulky building is worth saving from the scrap heap. The real issue is what should be developed in its stead.