I don't think much has changed as far as infrastructure goes. Maybe the flexible-pipe plumbing systems are different/better? Plastics and fake wood trim (both still common in many RVs) might be higher quality and less prone to fading/cracking, but that's a complete guess.
Otherwise, exterior and interior designs are what make up most differences. Slides make a big difference for interior feel, as Seiler mentioned. Most newer motorhomes have bigger wheels/tires and higher weight capacities as a result, compared to those from the 80s. Trailers are larger and lighter on average, but that's because they're built with cheaper/lighter materials. Otherwise new models have more creature comforts, like power awnings, LED lighting, outdoor kitchens, etc. that aren't really needed but a sign of the modern times.
Aside from some of those superficial feature upgrades, a well-kept RV from after 2000 (like my 2008 trailer) is very similar to something brand new. Manufacturers haven't really figured out any newer/better ways to build them. We plan to upgrade to a 5th wheel sometime in the next few years, and I'll probably be looking at 2010-2015 models. Whatever I have cash saved for, that meets our needs. No reason to pay the premium for newer, that I'm just going to lose in depreciation the first year that I own it.