Actually, I am surprised no one has mentioned a integrated electrical panel with a Electrical Management System/surge supressor. I bet this is the #1 mod most of the folks get (take a look at the Progressive Industries HW50C 50 Amp Hardwired Electrical Management System with Remote Display)
And while you are at it how about a 35' self-stowing shore power cable? (35' is NOT a typo...how many times has 25' just not been quite
It's 2014 - upgrade your old antennas to something like the Wineguard (and put a auto retract sensor on 'em for idiots that forget to lower them when they break camp
Speaking of the 21st century
How about installing a WIFI booster antenna on the roof with wired connection to the entertainment system that would allow the user to install & upgrade their own WIFI router? (I might want a Apple Airport someone else may want a LinkSys)
Upgrade all the TVs to WIFI enabled TVs (NOT the Samsung Smart TVs because technology changes rapidly!) Instead just TVs with multiple HDMI ports so a owner can install a Roku, Apple TV or Google Chrome device.
Upgrade the entertainment system's AM/FM/CD/DVD unit to play blu-ray (although for me, if I have a solid WIFI connection I will stream content)
Finally, it seems like a lot of folks put in a central vaccume system mod - while not very high on my list, it may be on others (I would be interested in it if it was a wet/dry system though!)