They said at the beginning that they still owned their house and were renting it out; assuming they didn't have a mortgage (which is possible), between that income and what they make as apparently pretty in-demand photographers, they could be doing pretty well.
This was a big letdown for me after the huge freaks of the series premiere, in that Tyler and Wendy seemed to be having an adventure that was kind of a pain in the kids' ass sometimes but that wasn't wrecking them. I think the producers deliberately followed them during a camping phase to make their lives seem especially rustic, and only for a couple of days (they cut up the photo shoot for two different out-of-sequence acts of the episode, which you could tell by what everyone was wearing and the fact that they didn't show the clients in the first bits of footage at all); sometimes they stay in hotels, and presumably eat in restaurants where fresh produce is available.
I agree with everyone who said the Whitacres won't be able to keep this up full-time much longer: it was probably no big deal when the kids were little, but I feel like Wendy, at least, will not get too precious about it and will realize that the kids need to go to real school if they're going to get a decent start for college, not to mention that Isaac will need to have access to regular visits with the same orthodontist. (Even in this episode she said she envisioned a future where they were only on the road half the year, so given the way she talked Tyler into the trailer, I bet her six months on/six months off plan will come to fruition before too much more time has passed.) They've clearly figured out how to handle the logistics (setting up a business model that works; dealing with all their banking and bill-paying and whatever), so spending summer vacations traveling while living in a permanent home the rest of the year would be a perfectly fine compromise -- just not one that would get them on a show like this!