Boondocking is easier if you pick your rig with that in mind. Fresh water and waste tanks will determine how long you can go between having to find someplace to dump and refill.
You will see threads about batteries, solar, generators and charging. You will need power while boondocking unless you find a older rig with a fridge that doesn't require 12v and use the rest like a hard sided tent. Read up on it a bit so that when you look you will know to look at how many batteries it can take, what kind of converter, if you go solar, what will fit on the roof.
Storage is at a premium in a RV. Some 5ers and Motor homes have basements but those will fill up fast if you are full timing. Just boondocking alone requires tools and back ups that you wouldn't need in a park. As a occasional camper we take enough clothes and towels, etc to last a trip. Full timing you will need clothing for all seasons including shoes, boots and coats. You will take things with you that we never would like paper work and mementos. So really look at what would go where when you see a rig.
Size can offer more storage and room but it also takes a bigger truck and may be a limiting factor as to where you can take it. Our trailer isn't as tall or wide as others, we use the advantage to go where others can't.
Last thing I would look for is a good size freezer. when you are out more than a week, it is hard to keep fresh food. Many things like milk, breads, meats can be frozen and stay good far longer than they would in a fridge.