We stayed off-site last September with DD4 and DS3, though we were very close (Sheraton Vistana, couple miles from DTD). Both of them were still routinely napping at the time, so it wasn't difficult to get them down for a nap (most times they fell asleep in the car on the way back to the condo). Our experience was that after 3 or 4 hours of touring, they were pretty wiped out and this made them more pliable.
We are going in May and will be staying on-site (BWV) this time, which should prove even more convenient for going back to the room for a break. Over the last couple months, the kids have been napping less and less, so we are anticipating that they may not actually nap during the break time. But even if they don't sleep, we believe getting away from the mental stimulation and physical exertion associated with the parks will freshen them up and place them in better spirits for the remainder of the day. All that said, you just really need to read the kid - if they start to get cranky or "zombie-like", then you probably need to do something to get them a break. If they have a lot of energy and maintain a pleasant disposition, stay.
One of the other things that we did was split up on a couple of days - one of us would take the kids back for the break while the other stayed at the parks. This was really nice!