A school bus is an alternative to your van plan. They are very much more available with diesel option, fairly cheap, way more head room. Can get one in a shorty flavor. Not as incognito as a van, true, but how often are you trying to park on a city residential street to sleep in it? Wal Mart doesn't care if you're parking a school bus there, or a truck stop.
To my knowledge, there were not alot of vans produced with a diesel option to begin with. Full time traveling in a van I can see wanting a diesel, but if you're only looking for an excursion rig to camp out of occasionally, a van with a Chevy 350ci small block will get the job done all day long. Can find older ones with a 305ci engine that will get little better milage if that's a huge factor to you. Newer (1997+) Chevy express vans can be had with the vortec engines that will get a little better milage than the precursor engine.
The sprinters w/ diesel are becoming more available for cheaper as they get older, and I've heard great things about them going 200-300K miles before needing any major repair. Not sure what your budget is, but getting double the milage in a diesel sprinter would be worth paying a little extra up front in my opinion, if I was set on a van I wanted to see good fuel milage out of, and have the head room. Just my .02 cent.