Couple questions on how Class C's are constructed.

As for wall construction, different manufacturers have tended to use different techniques for a long time; there's no one general time, so far as I know, when materials changed. (The same goes for the structural framing, as well--whether it's wood, or aluminum, or steel.) I personally wouldn't run away from a luan substrate if the RV is in sound condition.

The joint between the bunk board and the cab proper doesn't seem to generally be a major source of leaks. I believe typically these are assembled with a rubber gasket between the two, and screws holding the bunk board to the chassis cab roof. Presumably they tend not to leak partly because there's not much way for water to work its way in (since it's protected by the overhang) and partly because the weight of the cabover structure keeps good pressure on the seal. Much more commonly troublesome are the various seams at the roof joints, the running lights, and the front window in the cabover if there is one.

I don't understand why people think that 22-24 foot (short) Class C's are best.
The rather common 27 foot class C's have more comfortable rear bedrooms with RV queen bed and sufficient storage and cabinet space to keep clutter out of the aisle and shower stall etc. A few Class C's have separate bunk beds in the rear bedroom. Sleeping in the overhead or convertible dinette, etc. is OK for one person, short term. sleeping in the overhead is not so good with one person climbing over the other to go to the bathroom in the dark down the ladder and return without waking the other. Most Class C's have the same width and height regardless of length. Driving and parking a 27 foot Class C is not much different than driving a 24 footer and gas mileage is not much different. When you look at class C's of different lengths and floor plans, go through the motions of sleeping, eating, watching TV, cooking, etc. If rig has slides, are they in the way when retracted while stopped for lunch, etc.??

Having owned a class C before and not liking how much the cab over bunk blocks your view went with a 25 foot Thor Axis class A. It's on an E-450 chassis, V-10 6 spd tranny. Build quality is about typical for the RV industry, plan on time at the dealer for repairs. Do an in depth PDI prior to accepting the unit and maybe most items will be identified.

If your brother and you are going to tour the west, how much stuff are you planning to take? Will you tow? Class B & C RVs have weight limitations depending on the chassis underpinning the rig. One of the problems with the Transit chassis is it's very modest Occupant & Cargo Carrying Capacity (OCCC). In other words, at this point it can't safely carry much in the way of water, food, clothes, and stuff.

You might want to consider looking at a model built on a Ford E450 chassis, that is still relatively short, like 24 feet. A new E450 chassis has a GVWR of 14500 pounds, although some of the older ones carry less. This set up currently offers one of the highest carrying capacities around (depending on manufacturer).

Good luck with your search.

You might also want to check out equally well built Lazy Daze motorhomes, if you see a used one for sale by a private party. Lazy daze does not have slide outs, and that is a deal breaker for many. Lazy Daze only makes class C, and they are light, use innovations to keep weight down, storage in unique places, such as the bathroom door has a hallow space for a spice rack.

Many short RV's will have a 3/4 queen bed in the back, along with rear bathroom on the passenger side. Is this the size you are looking for?

I am not sold on the Transit motorhomes yet, their most powerful engine was not available in the cab and chassis for 2015, perhaps you can get the 3.5L Ecoboost in the cab and chassis now? But the 3.7L gas engine or diesel was the only choice in 2015, both are not rated to tow much more than the RV weight itself. I would much rather have the V10 in the Econoline, rated at 300 HP and towing up to 22,000 GCVWR.

I actually have a E-450 based shuttle van, it is 23' long overall. 16 feet behind the drivers seat, with room for 12 seats and 2 wheelchairs.

I also owned a 1985 class C on a GM chassis, that was 27' and traded it in on a new 1997 Bounder on the Ford chassis. I toured the Fleetwood factory twice before buying the Bounder. THey are well built, with steel sidewalls sandwiched into the fiberglass outside, styrofoam in the middle and interior wall.

I have not heard of complaints of water leaking from the cab to cabover area. I guess the only way rain can get in is while driving in a hard rain, and the wind pushing it into the cracks? Anyway not a problem with my 12 year old class C, or my shuttle van.

Good luck!

Fred.