I agree with what pretty much everyone else is saying...the RV doesn't HAVE to drive the cost of living. That said, certain things will be more expensive...insurance is an example. Clearly, if you want to insure the reasonable value of the RV, it's going to cost a lot more to insure one that is $600,000 than one worth $50,000. Maintenance is also a consideration. IMO, the basic maintenance on a gas coach is not too much different than on a full-sized gas pickup truck, if you do it yourself. However, on a DP, you generally measure your oil changes in gallons rather than quarts. Filters are much bigger. Transmission changes are a couple of filters and gallons of expensive fluid. The air filter on mine runs around $100. Many more grease points to take care of...if you do it yourself, not a big deal...but if you're paying someone to do maintenance...all those extra touch points are more time, and that's money. Generally bigger tires, and on the biggest coaches, more of them, which cost more when the time comes to change them out.
So to put it another way, I believe the "cost of ownership" of a DP is more, and all other things equal, you will need a little bigger budget. Other things not equal, like spending a few more nights boondocking, etc. and you could save that money.