Here are a few "secret" tips for Yellowstone lodging, or a campsite at Jenny Lake in GTNP (top secret, so shhhhh, don't tell anyone!>) ---
Tips for lodging in Yellowstone (not a campsite, but a place with a real bed):
Tip #1, as mentioned above (so not really a secret), is that people constantly cancel their reservations for YNP lodging (after all, no penalty if cancelled at least 24 -- or maybe it's 48 -- hours in advance). Hence, repeated calls to Xanterra can (i.e., definitely will) eventually get you one of those cancelled reservations.
Important corollary to Tip #1: those cancellations will happen even on the day that you arrive at the park, as we've found to be true by calling Xanterra as late as the day we drive into the park with no reservation having been made (one time, this "last-minute" tactic rewarded us with a cabin at Old Faithful Snow Lodge; yet another time it turned up a nifty room in the old wing of Old Faithful Inn);
Tip #2, once you have a reservation through Xanterra in YNP, even if it's not your first choice for location, that reservation can be transferred at any time for no charge to another location in YNP lodging if you want to keep trying to get your first choice. But it would be wise to verify, when you make your initial reservation, that this option is available if you intend to keep checking back for your first choice.
So long as the option to transfer is available, IF (actually, when) there's a cancellation at your preferred location, you have a good chance of getting it if you keep trying and trying and trying and . . . .
Illustration of Tip #1 & #2 at their optimal level of success -- Last fall, toward the end of an "extended stay" at a tent site in Madison Campground, temps got very, very cold and bad weather was coming. So we decided to bail out of the campground a few days early -- before our "extended" reservation period ended. Rather than just get up and leave, we called Xanterra to ask if there were any lodging cancellations "as of that very same night" at Old Faithful. Yep, someone had just cancelled, and we slept that night (and the next couple of nights) in a bed at Old Faithful.
Bonus: Xanterra transferred the remainder of our reservation at Madison campground to our last-minute lodging reservation and adjusted the lodging price to credit what we had already paid for those last few days at Madison campground.
Moral for lodging reservations at YNP -- get your foot in the door by making a reservation, then keep on checking with Xanterra for a cancellation of YNP lodging at the location where you really want to stay, right up to when you arrive to check-in.
Tip for Jenny Lake Campground in GTNP -- Jenny Lake Campground has several walk-in sites set aside for foot or bike travelers. If you check those sites in the late evening, may find a couple of them that aren't going to be used. If so, you're set for night #1.
Before turning in for the night, though, take a walk through the car-camping loops to see which ones will be vacated the next morning (that info will be on a piece of paper clipped to a little post at each site). Pick the soon-to-vacated site that you like best; go ahead and get a registration form at the campground entry; and early the next morning, go the campsite to confirm it'll be vacated soon; if it is, fill out your registration form and clip it on the post (underneath the form belonging to the soon-to-leave campers at the site, but checking with them first to be sure that's ok with them). Then go back to the walk-in area, strike your tent, and go get a hot cup of coffee at the Jenny Lake store while you wait for your campsite to get vacated.
If all else fails in GTNP, and its getting dark, there's the Gros Ventre Campground to check out. Seems like there's always an empty spot there.
For Yellowstone, the "last-ditch-end-of-the-day" option probably requires driving outside the park. For sure, though, try for one of those very-last-minute cancellations, and start calling early in the morning of the day you need a place to stay. Keep calling, too. Xanterra reservation operators are real nice, and also patient. They must be used to it.
If you just want to avoid leaving your "extra" gear in your car at a trailhead while you spend several days backpacking, one option would be a nearby short-term storage unit.
For backpacking in GTNP, or in the Winds, there's the Storage Stables (reasonable rates and flexible short-term rental periods) on the southern outskirts of Jackson, along Hwy 89 (the highway to Pinedale and trailheads for the Winds). Here's the link to its website --
The above storage facility is real convenient for the Tetons, and also works well for the Winds so long as a return to Jackson Hole is part of the plan after backpacking.