To the OP, Class Bs are wonderful if you don't require a pricey rolling condo with fireplace, washer-dryer, multiple bedrooms, etc. As a former Casita-owner, you sound like a good candidate accustomed to smaller space advantages. We still miss ours and will no doubt buy another one in the future.
They are more expensive than people expect, but they are much more custom-fitted, often with more durable quality materials, and have stronger resale value than most RVs do, too, so you'll probably get a return on the other end. We did.
Easier to find both new and used now that many more models are being produced. You could do some preliminary online searches to see what some of the big dealers like Lichtsinn RV in Iowa or PPL in Houston carry.
Some vans have raised roofs or lowered floors to allow more head room, while others like the Sprinters are entirely fitted into the existing steel frame. You won't have multiple exterior storage compartments, and storage is at a premium inside, too. However, the interior nautical-style compartments are well-planned and there's often a hanging locker.
Typically have shore power for 120, some have solar options. You'll find variations on small enclosed bathroom layouts, some with wet shower. Usually will have a galley with sink, propane cooktop, small three-way fridge, and microwave oven. Bed styles may be fixed twins, corner beds, or sofas that convert to a bed. Front cockpit seats may or may not swivel around to provide seating facing the living space when you are parked. You'll have fairly small fresh water and holding tanks, but can probably go three to five days depending on variables. Many have propane generators, propane/electric water heater, propane furnace. One or two house batteries. 12-volt outlets to use for fans etc when dry camping.
You'll find gas and diesel models made in the US and Canada. All get better fuel economy than most other category RVs. The T1N Mercedes/Dodge/Freightliner Sprinter diesel RVs (produced up to about 2007) get great mileage, later Sprinters get very good mileage. More recent entries into the RV conversion market are on Ford Transit, Chevy Express and Ram Promaster chassis.
Some names if searching for Class B RVs (in no particular order) are Pleasureway, Leisure Travel Vans (no longer selling new), Great West, Roadtrek, Coach House (made in Florida), Advanced RV, Airstream Interstate, Winnebago, and Sportsmobile which makes custom vans.