Let me answer in multiple parts, first off that class C you linked to may be a great find in the current market, being stored indoors is a BIG plus as many (most) RV's die from rot caused by water penetration around the roof and windows. That only leaves mechanical neglect items like dry rotted rubber components, and other maintenance items. It also leaves the big question of is it the right RV for you and what you plan to do.
I personally don't think there is any wrong class of RV's, it all depends on where, how and in what level of luxury you want to travel, at this point it becomes a question of frame of reference and your preferences. Are you comparing an RV to a tent, a motel room, or a house each frame of reference can be valid, and all RV's will be more comfortable than any tent, and less comfortable than nearly any house. Like many here I started out tent camping, then moved up through a series of a couple of camper vans, spent a few years without any type of RV, seriously considered buying a smaller travel trailer when my daily driver was a pickup truck, then eventually bought our current 28 ft class A coach about 5 years ago, when it was 14 years old.
This coach works great for our type of travel, which is up to about a month at a time on the road, though I am not sure I would enjoy full timing in it. You ask what fills up cargo space, the answer is everything, tools, clothes, food, camping gear, etc. My 28 ft class A has somewhat limited cargo compartment space, I will quickly give an overview of what is in each compartment which vary in size, from a roughly 6 ft long double compartment down to ones with only 12-18 inches of usable space due to water tanks.
Compartment 1 about 28 inches wide: Folding camping table (24x48 unfolded size), small propane BBQ grill, small propane firepit (Camco Little Red Campfire), 3 folding camping chairs, a 7x7 roll up camping rug tucked across in the tiny pass through space, propane extension hoses, and a folding step stool. All of this has to be arranged in a certain way to get the door to close.
Compartment 2, smaller, on most space is taken up by a fresh water tank, leaving just enough room for a 28 inch Dewalt rolling tool box like
DEWALT Tool Box On Wheels, 28-Inch (DWST28100): Amazon.com: Industrial & Scientific with just enough space left over for a few wood planks to go under the leveling jacks and folding roadside emergency triangles and collapsing orange cones.
Compartment 3, slightly larger than #2, milk crate full of engine fluids (oil, antifreeze, transmission fluid, etc.) a pair of 6 ton jack stands another bag style camping chair, electric impact wrench, sanicon tank buddy (macerator dumping pump for dumping tanks into a sewer clean out or similar), spare water hose and a few odds and ends.
Compartment 4, rear trunk area, this is mostly an access hatch but has a 5+ ft long by about 8 inch wide and 12+ inch tall storage shelf. This hold a mix of longer items, and small stuff. Telescoping squeegee pole, water flow through brush, emergency wheel chocks, spare safety cables for towed car, LED work light, spare sewer hose.
Compartment 5 and 6, these are mostly taken up by the black tank and another fresh water tank, plumbing manifold, etc. Though there is a little room in here for sewer hose, and fittings in one and fresh water hoses in the other.
Compartment 7/8 double wide compartment which is also the electrical bay containing the converter, surge guard, inverter and hard wired power cord. Contents of course include extension cords, power adapters, as well as a telescoping ladder, genturi ( smoke stack for generator often required at RV rallies, etc.) and a pop up CLAM Quickest canopy (largest single item we carry, just barely fits)
That is it for the storage compartment, and yes I may have left a few items out, like the mosquito candle in compartment 1, or the tire covers that came with the coach tucked into the back of compartment 3. Indoor storage is similar for example there is an ottoman storage box behind the front passenger seat (which swivels) that my wife props her feet on when she sits there in the evening. Inside it is a mix of stuff including a spare fresh water pump, a pair of binoculars, spare power adapter for the notebook computer, a small emergency tire inflator compressor, etc.
p.s. here are a couple of photos of our coach, at under 30 ft it allows us to travel many places without pulling a car around, over 20,000 miles in the last 5 years averaging over 7,000 per year until covid hit, with only about 1,000 towing a car around.