I have never heard this request before. You would be best to just talk with your sales person to see if they would install an RV toilet you supply. Their sales staff is very helpful with letting you know what the can and will do, and will ask if it is a special request. Plus, they know better what they can or will do better than the bunch of us.
Do let the forum know what you find out.
Is there a particular RV toilet you are interested in? I don't have much issue with the one installed, but am always keen to learn other folks thoughts.