I tend to drive mostly in the western US, specifically Colorado, Wyoming, Utah, the Dakotas, or Nebraska. Where are you trying it?
A while back I read a fun article about how online booking contracts were "unchristian". The christmas story sees a hotel manager make available a sub-standard room at a discount rate to a desperate walk-in family. Do that today and travelocity will be on your back about not publishing that "best rate" online.
Why not go with "we don't want you here", instead of something they know is a lie?
Anyway, I'll bet I'm turned away from ad hoc hotel stops 10% of the time at most.
Just curious: where exactly do you pitch a tent in a situation like that? It doesn't seem like it's that easy to find some open ground where some cop won't harass you for trespassing or something.
Alternatively, if you're in a U. S. National Forest and can find a spot off the road, you can camp anywhere you want. It's how we do the majority of our camping in our VW Westfalia, where in Washington there are a plethora of spots 90 minutes from our house (no, I'm not telling <g>).
But that's for my mode of travel, which is usually off the beaten path. If you're looking for a place to stay in SF during WWDC, my advice will do you no good. OTOH, I rode to a conference in San Diego, had some room trouble and found a campground smack in the middle of SD a few miles from the hotel. So, I dunno, there might even be a campground in SOMA I don't know about. :-) (EDIT: curiosity got me: there's one out by Candlestick Recreation Area, if you don't mind paying $89/night!)
The problem when I'm booking hotels is that:
1) I'm driving a long distance west of the Mississippi, and the stop points are very discrete and far between--so everybody wants to stop at the same point. For example, Austin to San Diego--El Paso is a bit short for the first day, so you go to Las Cruces which is a lot smaller and more likely to have fully booked up.
2) I'm doing a tourist thing which puts me in with a lot of other people.