Absolutely this. And, it can't be emphasized enough if you want to keep your expenses down.

As a preference, I think that this guy being able to make van living work is awesome. That being said, it's not necessarily cheaper than regular living unless you're willing to adhere to a certain kind of lifestyle. Awhile back, I looked at van living and there's an amazing community on YouTube of all places that chronicle the practicalities of van living and they talk about everything like expenses, sex life, etc. As one example on the expenses:

https://www.youtube.com/watch?v=bXA8_YeMvAA

Anyway, as much as everyone thinks that you can just park at Walmart every night, that's not necessarily the case or desirable. Not every Walmart allows it. After awhile, you don't want to live there. That leads people to guerrilla camp, which there's an art to. Industrial areas aren't always safe as there's no one around in case something happens. If you park in front of someone's house, they probably will call the police because it's weird. As such, the best guerrilla camping spots are in mixed commercial/residential areas where you can park on the commercial side of the street and still be near people for safety. As long as you move the van before the business opens, no one really cares. However, that moving around increases living costs. Loaded vans get terrible gas mileage. Gas isn't always cheap.

Alright, so we can just park in National Parks, right? Not always. Many, if not most, National Parks require camping fees per night, but it depends. Also, with budget cuts in Congress and not everything being fixed since sequestration, many parks have shut down their camping spots between Labor Day and Memorial Day, so there are fewer places to go. BLM land can be a bit more forgiving, but, really, because BLM doesn't have the money to patrol the amount of land they have and this only works in areas of the country with huge BLM controlled areas.

Alright, so say you do want to do it legally. What's the cost? If you ever want to shower, the cheapest place is a Planet Fitness membership at $10/month and there are many around the country. As far as rent, trailer parks vary in price, but in smallish cities I see them going for $300-$350/month. As far as food, since there's little storage place and refrigeration, expect to eat out quite a bit more or buy in smaller quantities, so more money.

As an aside, cooking is sort of a pain in the ass because cooking in the van will vaporize the fats in the food. That will, in turn, get it all over your stuff, which sucks and is a pain to clean. I wish I could find a link to it, but there was a video from one guy who did a beautiful build out of a van where he created an isolated kitchen in the side door of the van. In that way, all of the cooking problems were isolated from the living area with the consequence of having to go outside the van to cook, which made it more difficult to guerrilla camp.

As another aside, if you're unwilling to guerrilla camp, I came to the conclusion that it's better to just have a trailer that's towed. It tends to be more comfortable. When your vehicle breaks down, and it will break down, you still have a place to live while doing repairs unlike a van or RV. Also, the startup cost is much cheaper. I was at a maker fair this last weekend and this guy was there showing off a trailer:

http://camperplans.com/

$4500 for a full build out and all he started with was an axle. The result seemed pretty good, but I'm not an expert. I believe he sells plans for it as well. Certainly, trailers require a vehicle to pull them, but a small trailer like this can get away with something like a Tacoma and a used one can be had for a few thousand dollars.

Anyway, I think mobile living is amazing and workable. However, I wouldn't necessarily call it cheap. You pay for freedom, but it's a workable amount of money.