I'm not sure about the older porcelain toilets in the earlier trailers, but on the approx 70's and up, no, a regular toilet wouldn't work, as the flange to wall clearance wouldn't allow a regular toilet to fit. You also would need to figure out some kind of flapper valve to keep the water in the p-trap while you drove, not to mention not enough vertical room for the water tank.
The new toilets (and old star-magic thetfolds) come with a rubber flange that seals between the holding tank and the toilet. It all comes with the new toilet. Search for "toilet replacement" or "Bravura" to see my photos of toilet replacement in a 75 Argosy.