There's a fine but important distinction between drycamping but not in the boonies, and drycamping in the boonies. To me the "boonies" means drycamping kindof out in the middle of nowhere and maybe in a bit more tricky to get to - and maybe a bit tight when you get there - places.
IMHO, what this can translate into regarding RV type versus tank size is this: Get the smallest RV you can tolerate out in the middle of nowhere while at the same time equipped with the longest-time-frame self-containment-sustaining systems possible.
We have only a 24 foot Class C but on an over-kill chassis with respect to it's weight for additional mechanical ruggedness and reliability, along with over-stock size tire heights for a bit more ground clearance. It's rear end is raked up a bit to reduce tail dragging crossing road depressions. It's capacities are approximately 40 gallons fresh, 40 gallons black, and 30 gallons grey. It has all the comforts of home rain or shine, albeit with not anywhere near enough floor space to play bocce ball should we ever want to.
Since we have about 2250 lbs. of additional weight we could safely carry, I suppose fresh water capacity could easily be expanded to well over 100 gallons using a tank, or tanks, mounted up between the frame members alongside the driveshaft. Even under the dinette seats 50 more gallons of fresh water would probably fit just fine. Grey water can easily be off-loaded into the black tank to extend grey capacity while camping in the boonies.
IMHO, a Class A makes a superb drycamping-only rig ... but a way too restrictive drycamping in the boonies machine due to it's height, the length of it's wheelbase, it's rear end departure angle, sometimes it's length, and usually it's ground clearance.
I'd recommend starting your search for a boondocking RV with a small Class C or a large Class B, with the most important criteria being large capacity tanks and a quiet built-in generator with both ducted heating and ducted air conditioning for best flexibiliy on when and where you can go in complete comfort. Just about everything else beyond those things would be gravy.