A B is a van, the size of a van, not matter how you look at it with a narrowly resembling a real bathroom in it. They are great for a couple weekend camping. But a 6 month trip with family? I think you will be very unhappy. Money spent on a trip that no one had fun on. All for the sake of 1 or 2 more mpg?
I have a 31 ft V-10 and I get 11 mpg. Got 12 a couple of times but that was before I stared towing.
I serious try to keep it at 55 mph which seems to be her sweet spot for mpg, going any faster than that only gets you to your destination just minutes earlier and with a how lot of sucking gas IMHO unnecessarily.
Keeping your C And not having new RV payments is IMHO priceless. Sounds like you would be adding payments$ for a new RV 'just' for a 6 month trip? And then it will sit while you are paying on the new one?