It still has the exterior door panel just like the tank-type but is a much shallower installation depth and does not have the thermal mass of 6 gallons of hot water waiting at temperature. The tankless does have a winter feature that will cycle the heater based on water temperature so it prevents freezing. I can post a picture of mine since it is sitting here in my driveway.
I would say that a tankless would not be ideal for use in consistent freezing temperatures. We camp from April to November, and it hasn't been an issue for us, but I can certainly see where it might be for some.
For what it's worth, I had picked up a brand new Dometic 6 gallon DSI gas/electric water heater and was planning to install it when we took delivery of our Cougar, but we started camping almost immediately and time got away from me. Before we knew it, we had gotten used to the tankless, which again for our camping style ended up working better for us than the tank-type so I sold it.
As long as there is enough depth to accommodate the tank type WH, it should fit in the same opening without modification. The power and the controller circuits for the gas and electric may be another issue depending on the model/floorplan. I was able to locate the bonded ribbon of 4 control wires on mine (they still ran the wires but didn't connect them to anything), so it would have been simple to wire a tank type in place of my tankless.