Any accountant will tell you it's an easy answer - staying in hotels is cheaper.
However, there are variables you would have a hard time putting a value on:
Flexibility in your plans - Many times I can't get a hotel reservation, so I either have to cancel or stay in some dump I didn't plan on. I've rarely been turned away from an RV or state park. Some places need reservations, many hold spaces open for drive-ins.
Knowing your bed and bathroom are clean.
Preparing you own food. Controlling what you eat, what is in it, and when you want to eat.
Carrying more than you probably would - having clothes for variations in weather. Bringing a bicyle or fishing gear to use if the opportunity presents itself.
Making unscheduled changes in your itenerary. Deciding to make an extra stop along the way, or staying a few days longer.
It can be less work! Keep your RV stocked, you can go at a moment's notice without all that packing, unpacking and luggging suitcases up 3 flights of stairs.
All these options are hard to put a price on. I have a high value of my time, so being in control of it is very important to me. I rarely fly these days because of that.
Now there are plenty of ways to save money though. Smaller is definitely better - don't buy more RV than you need. Most of the monster RV's sit in storage because of the cost to operate and the hassle/work of traveling with them. You can save money slowing down - the faster you drive, the more you spend. If you are out west, boondocking on federal land is nearly free.
I'd suggest that if you are just getting your feet wet RV'ing, to buy a very small RV - like a Casita, or a T@B, or a teardrop of some sort. You can minimize your costs, most likely keep it at home, and see if you will use it frequently. If it works out, you can always upgrade accordingly. If it doesn't, you can get out and cut your losses.