well, Tarifa's not that crowded yet in June, at least by european standards, you should go there in July/August - now that's crowded.
If you're travelling by RV to save money - great idea, just keep in mind that it's illegal in most european countries to camp on your own, so if you don't want to risk paying some pretty stiff tickets, you're limited to public camping grounds, and they can be pretty expensive during the high-season June thru August, especially RVs.
I'm too lazy to figure out the european equivalent to 22mpg, it's pretty good though, but in case you didn't know already, one liter diesel averages about 1.10 Euros, about 1.30$. If I'm not mistaken that translates into 5 bucks a gallon. Did I mention the stuffed wallet? Now you know why.
I've never been to Gotland, but I hear it's awesome, the women there are just... wait a minute, this is a kite forum, isn't it?