If we are still talking about the Majestic Tour Class B?
Then the generator on mine runs off gas and not propane. ( it will run until your tank gets down to about a quarter of a tank and then will shut down.... its a safety thing.)
As far as adding more batteries? The coach battery is located under the vehicle kinda towards the rear, but you might be able to expand that area with a little thought.
I would think relocating the coach batteries to a box on the trailer hitch might be a better option?
Solar is always good, but if your in town most off the time and driving your van as a daily driver... Then this would keep things like your batterys up.
So the solar would be that little extra you could do later if you where planing on being off the grid for a few days or weeks at a time and not driving your van much at all.