I replaced mine with one I bought at Best Buy. They have a pretty good installation team that would have done it for a reasonable price. I am a retired electronic tech so I did it myself but look at what Best has. They had the adapter to adapt my new stereo wires to the existing wireing harness. It was just plug in I did not have to splice any wires. The original radio in my Allegro Bus was made for a Ford Product.
If all you are wanting to get is one for navigation. I prefer the separate GPS. For one thing you can get those that are made for RV's they allow you to put in your length, height, weight etc and monitor your route for things like low bridges. The other reason is for spousal navigation. If we are looking for a restraunt, campground, alternate route, etc. I just hand the GPS to the spouse and she can do that while I concentrate on driving.