Many people are doing this now, thanks to the wonders of deals like the Amazon Affiliates program, which gives you a small cut of anything purchased on Amazon if the buyer first clicks through your personal blog.
As for the example you cite, I've long been puzzled about that couple. They must have a trust fund or something to be able to afford a very expensive RV like that right out the gate, not to mention a brand-new toad. In addition, I believe they replaced the RV with another brand-new RV after just the first year or two - just because they wanted something different. I'm not criticizing -- to each his own -- just saying they probably do not make all their money with their blog/videos.
Here's another such RVer I admire who is probably a more typical real-world example (including links to detailed monthly budgets):
http://rvsueandcrew.net/
