Differences will mostly be in the chassis, rather than house construction and appliances.
If Ford, 1997 (chassis model year) gets you into the fourth generation Ford E-series with Triton engines, some engine improvements over the years and chassis improvements around 2001 (E-450 or E-350 Superduty) and 2008 (front end restyling brought front axle upgrades).
If Chevrolet, 1996 gets you into the Express/Savana, GM's first body on chassis van series, replacing the Chevy Van. Engine upgrades from classic small-block V8s derivative from 1955 and L29/L18 big-blocks to the new 4.8/5.3/6.0 small-blocks came 2003 model year, with six-speed automatic for the 6.0 starting in 2010.
Also, 20 years will get you into Dodge platforms for Class C, although by then they were fairly rare, as Chrysler was supposed to get out of the medium-duty truck and motorhome business as the condition of their Federal bail-out in the mid-1970s.
Go back more than 20 years, things are more different. Ford chassis C motorhomes will be using the Windsor V-8s from early 1960s or the 460 big block from the late 60s, and the further back you go, you lose things like fuel injection, computer-controlled transmissions, and far enough back, overdrive. Similarly for Chevrolet, when pre-Vortec you lose port injection and go back to throttle body injection, then carbs, with similar issues on transmission tech.
House construction changes over time vary more by manufacturer. Winnebago started using laminated panel wall construction in the late 1960s, other manufacturers were still doing stick-built houses into the 1990s. A couple of specialty manufacturers still build with wood and steel framework, and probably do a better job of it than mass market manufacturers assembling RVs from laminated panels.
So there is a lot more to this than age. If you find something 20 years old that was built exceptional well and taken care of, it might last longer than anything you can buy brand new.
We are just now seeing major changes in type C motorhomes, with the introduction of global market chassis like Sprinter, Transit, Ducato/Promaster with lighter capacities, new appliance and systems technologies in the house, and smaller sizes and lighter weights for substantial improvements in fuel economy and thus operating costs.
The Ford "spitting sparkplugs" issue is a 1:100,000 to 1:1,000,000 problem across the total production of 5.4 V8 and 6.8 V10 engines installed in pickups, vans and SUVs, and tends to be related to the second set of plugs installed at 80,000 to 100,000 miles. It is worse on vans than pickups and SUVs because the mechanics replacing plugs find the rear plugs on vans more difficult to properly install.
