Since you are asking about full-time, it is your only home, then what is best is a complete house on wheels with two sleeping areas, preferably without having to convert a living area into one of the sleeping areas.
This gets you into a C motorhome 28-30 feet long (assuming you would use the overhead bunk) or a A motorhome 34 to 38 feet long, or a 27 to 30 foot trailer (thus 25 to 28 foot box). For the A and the trailer, those are the sizes where you first start to find separate front and rear sleeping area, though it is quite feasible if you are willing to do it every day, to put the children in the bedroom and let the adults sleep on the sofa in the living room. Much of the world outside the U.S. still lives that way in there permanent homes, where 600-800 sq ft is a fairly common family living space.
An A more expensive than a C? Not really, if you are shopping used, and not shopping for the biggest and most luxurious thing.
I think towables work better for families that are moving from place to place but staying in each place for a considerable time, several months for example. I meet a few families doing this, moving for work on seasonal or short-term jobs (like agriculture, construction, infrastructure repair, seasonal warehouse or sales work).
I think motorhomes work better for families constantly on the move, because you are always in the house, children not spending most of their day in a car or truck. They also get used for longer term stays, but that means towing or driving a second motor vehicle for getting around when the motorhome is parked. In my experience, that adds to the cost, though it can be less expensive if the second vehicle is a low-cost disposable car, which may need only liability insurance and doesn't have a lot of money tied up in it.
What it costs, what works best, how it works, depends on where you want to be with your lifestyle. Full-timing can be done on a $2000 a month budget and it can be done on a $20,000 a month budget. It can be done starting with $20,000 to $100,000 for your equipment, or you can start out at $2,000,000 for a rolling mansion. I've seen people get started on even less, an under $4000 RV, but they also seem to be constantly spending that over again to keep going.