Depends on whether the lifestyle suits you. Some people love it, some people get tired of it quickly or after a year on the road, and some hate it right away. If you've never tried RVing with your girlfriend for an extended period of time, definitely set up a "pilot" or "test" trip before you start going all in on selling your house, purchasing an RV, etc. A lot of people have this fantasy and discover that the reality does not match expectations. Go on forums meant for RVers - read, ask questions. Not everyone is a retiree or over 70 years old.
There are also a lot of choices about the best vehicle set-up, and it's not clear why you selected that one. Your biggest cost on the road is FUEL. For full-time RVers, fuel = rent. The way to RV cheaply is to travel very slowly, not to cover lots of distance. You linked to a gas guzzler. There is also a major choice in what set-up, because your lifestyle/plan matters for whether you will want or need to bring along a car (toad), motorbikes, or bicycles for when you aren't driving a 30' + vehicle.
In terms of your financial plan - the t-shirt thing may work. Have you looked into permits and permissions you will need to be a vendor at these events?
You may be able to turn your adventures in a blog/book, but what is your niche with this? There are quite a number of blogs out there from people who full-time in RVs (google it).
Chris and Cherie at technomadia just celebrated 10 years on the road. There is a whole community of RVers who are young and working/semi-working on the road. For example:
http://www.technomadia.com/young-full-time-rving-nomads/Their site has a lot of excellent resources for how to get started. They also post full budgets (as do a bunch of other fulltimers) that can help you get a sense of costs/earnings involved. Many of these people are frugal, DIYers, and similarly-minded about values and lifestyle as the MMM group.
This isn't a good or bad idea. The good news is that there are tons of resources for you on this, and it's pretty easy to get 'started' trying it out.