You're going to have to go Model within Brand. By that I mean that the storage rating, in cubic feet, isn't constant within the product lines. You also have to be sure the most storage comes with an interior floor plan that you can like. For examples:
A "Full Basement" Class C is going to have the house floor at least two steps above the entry, and over a foot step up from the cockpit. Those climbs can be daunting for some people. Also, a high house means little to no view out the windshield while camped.
Is the SHAPE of the storage useful? Our coach is NOT basement, and has a goodly amount of storage compartments, but none is tall. They'll take boxes, long tools, buckets, but nothing like a BBQ or a 5-gallon paint pail.
Does the storage take something you need out of the interior? Some models enlarge exterior storage by raising the bed. Is that OK or too high? Others do it by making a split level interior. That's a tripping hazard for some. Is it OK for you?
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB