With regard to vans/minivans and using them to sleep in. My only concern is gas milage. I am not familier with what kind of gas milage a mini van might get but if the milage was to poor, could that offset the savings. In other words, in the end, would it be cheaper just to get a cheap hotel. Of course, regardless of cost, the flexibility of being able to just sleep about anywhere certainly has some monetary value.
There is a certain joy in exhaustion.
Our '1991 Ford Escort Service' cost $300 at an auto auction and gets about 32 mpg and our '1988 Honda Hilton' was purchased for $350 (from a Sunni Muslim) and gets about 35 mpg.
We probably have slept in them at least 50 nights each.
Motel cost for 50 nights (2 people) - at least $2,000
Our cost - $650 (with two mechanically sound cars included).
A $1,350 savings means at about two less weeks of working and two more weeks of walking.
It is all about choices and your comfort level.
This wouldn't be considered cross-country, but AT trailheads can be good places to park and sleepover for free. When I drive all-night from G'ville , Fl to the AT in GA/NC/TN some of my favorite free camping spots are (1) Rock Gap Shelter, GA, a coupla 100 yds right off the road, (2) Roan Mountain Shelter about a mile uphill from Carver's Gap (3) Albert Mountain right off the FS road in Standing Indian (4) Snowbird/Wildcat Top (this can be a fun drive in a 2 wheel drive vehicle). My favorite is Beauty Spot about 100 yds uphill from the parking overlook. There are many more.
I'm not really a hiker, I just play one on White Blaze.
In 1990 I drove from NY to CA and back via AZ in a 1984 Chevy Caprice Classic station wagon. The old V8 full sized kind. I did a lot of backpacking along the way, and in four months only stayed in a motel once (girlfriend visited). I had a couple of old poly sleeping bags and blankets and would set up in the back if I had to sleep in the vehicle. I developed the technique of simply pulling camping gear over me as camoflage if I was concerned about being disturbed. I did not sleep anywhere illegal, and found a few situations where I got permission from the landowner to park and sleep on the ground next to the car (farmers). I generally looked for small private campgrounds if public land was not available. I found that I could usually negotiate a low fee ($10 or less) if I explained that I was using a very small tent, made no fire, used no electricity, and would help split wood or do other chores. I found quite a few "mom and pop" operations where they essentially had people camping behind their house. Swinging a maul saved me a lot of money.
The best bet, as Grey Blazer indicated, was to find a trailhead for a state or national forest and hike in a bit and find a good spot to sleep out. I had a very small tent (TNF Mayfly) which is a glorifyed bivvy sack which can be set up and broken down in seconds. National Forests are very permissive in camping. National Parks are more restrictive, crowded and expensive. I generally avoided "car camping" in any National Parks because of the noise of generators and drunks. You should learn the regulations in force at each type of area, and it is always good to ask the locals for advice.
I never had a problem. I had a few instances when police or rangers checked on the vehicle with me in it. If I was awake enough to talk to them they just wanted to know what I was doing in the area. Hiking and birding was all they needed to hear. A field guide to the birds is very handy when contacted by ranger-types.
I traveled from July through October and the whole trip cost me about $2000 including some equipment, food and gas (in 1990). I did not modify the wagon, except to remove the back seat cushions to make more room for gear under the deck when the seats were folded down. The deck was perfectly flat when the seats were folded down. I rigged clotheslines inside the back, and anytime I was driving between destinations I had clothes drying in the back. The biggest problem that I had sleeping in the vehicle was getting fresh air without letting mosquitoes in. I tried to rig netting over the windows using velcro, but never had much success.
Good Luck!
Another way to car camp is to get the AAA camping books for the different states and circle all the free campsites in the book and put an asterisk at the top of the page.
On one of my western car camping trips, I had circled all the free campsites that were at 8000' or more. Found some really remote beautiful spots for free.
I'm not really a hiker, I just play one on White Blaze.
We travel a lot. We've slept in the back of our Pontiac Vibe (the seats lay completely flat) or in the bed of my truck on many occasions. We have slept in parking lots, usually of hotels or of rest stops along the interstate. We have also slept many a night in our tent on National Forest land. My understanding is you can set up anywhere on National Forest land. We have also set up our tent in parks or campgrounds. They usually charge a small fee, but we typically arrive late and leave early, so no one is ever there to collect the fee. We've found that many forest service campgrounds do not have after hours collection boxes, so we have no way of paying unless we wait around. If there is a collection box, we pay our share and register. We've also slept just off the sides of roads, but never on private residential property, and always far out from any resemblance of civilization.