Most?
There are far more off-property rooms than on-property rooms. Also, a number of folks just drive to WDW as a day trip- no hotel.
Also, a number of on-property hotel guests stay at a WDW because they are attending a work convention. Though that is also true for many offsite folks as well.
What = an offsite hotel? I have myself driven from Tampa area to WDW to visit a WDW park, but I wouldn't really say Tampa = the WDW area. (I've also done Tampa hotel to Universal, and even further than Tampa direct to WDW.)
The Orlando Sentinel puts the area count as over 126,000 rooms (2019), and 5 million sqaure feet of convention space. Las Vegas Sun say Orlando has over 144,000. WDW has approx 36,000. (Magicguides says, "In total, Disney World has over 30,000 hotel rooms, 409 wilderness cabins, 799 campsites, and 3,293 DVC units.") Universal has 6,737 rooms. - though this is just what I found w/a quick internet search.
When we say people are staying onsite for the onsite perks, are we including business-convention goers? I don't think they care that their convention is in Orlando instead of Vegas. They don't really care about Early Entry to Epcot.