At 15-20yrs old, you have electronic fuel injected engines and slide outs available, so no worries about a carbureted no slide rig.
Yes, there are some improvements in power but unless the rig was underpowered to start, it shouldn't be a bid deal.
Appliances are pretty much the same as are construction methods with the possible exception of a move towards more fiberglass siding units as you go newer.
There may be some advantages to the stereo/TV but those are cheap and easy retrofits and even new systems aren't typically high end.
If you want auto leveling and similar goodies, they can be added to an older unit just as easily as to a new unit.
If the rig is in good condition, there really isn't much difference.
PS: The 10yr old rule at most parks is only to keep out run down pieces of junk. They rarely enforce it as long as the rig is clean and in good condition.
