I've got a coworker who did the whole drive to the Grand Canyon road trip in January a couple years ago. She said is was practically devoid of fambiles, and the only real problem was driving through the Rocky Mountains, which got kind of scary during a snow storm.
I'm still advocating for the week after Labor Day here...I live near Wisconsin Dells, a wonderland of tacky tourist traps, amusement parks, cheap teeshirts and fudge in case you don't know. Anyway, I have to visit it every few weeks for work. To compare the experience pre-Labor Day, with a post-Labor Day experience is like comparing Chuckee Cheeze's to Chez-Too-Goddamned-Expensive-To-Bring-Kids-To-Even-If-You-Could-
Afford-The-Extra-$100-For-Their-Wasted-Food.
I figure, if enough of us take our vacations at a time which might coincide with a time when fambilys for the most part aren't around, there'll be enough of us giving the stink eye to the parunts who do decide to take young Toadly to experience the grandure of (pick your destination of choice), that they'll think before they take Toadly out for the unappreciated vacation at that time again. Of course they are parunts, so I doubt they would get it.
If a bunch of childfree folks go on vacation at a time when the folks who make money off tourism are still open, but not making money off famblies, and tell them how much we like their facilities without children, they might actually offer us some free meals, cheap tickets, lodging deals etc. to try to make money off us when the famblies aren't around. I just wish there was just a week when I could go anywhere and not have to worry about some screaming, filth spewing, germ factory running into me AND get offered some free crap to lure my "all my income's disposable" butt into their place of business. I know there are "adults only" resorts, but they seem like sort of a creepy swinger, key party sort of thing. I could be wrong about that, I guess.