No...do NOT install a p-trap
That would only leave waste standing in the hose to permeate it and cause odor problems instead of curing them. Odor coming back into the bowl is caused by two things: incomplete flushing--stopping as soon as the bowl is empty, leaving waste standing in the hose, and a joker valve that needs replacing. Before you begin this project, I strongly recommend that you read the articles in the Head Mistress forum Reference Libarary (on the forum homepage. Also a very recent thread called "How To Order Ronco Tank"...it digressed at the end into conversation about where to install tank fittings that you might also find useful.
