"He who marches out of step hears another drum." ~ Ken Kesey
New Buffalo has one - http://www.newbuffaloshirt.com/Locations/NBSFRoaming
If the main thing you want is mobility at, say, different events, but don't want to commit to pre-printing, then heat pressing plastisol tranfers is probably the easiest thing, with the least investment on possibly unsold shirts.If you like the gimmick of printing them to order, then you still shouldn't worry about burning screens on the spot.A small compact 4 color manual, a flash, and a five foot dryer could get you by and be fairly space efficient.That, along with shirt storage would all depend on the size of your trailer.
We looked at concession or game trailers where the sides opened. Thought about a 4 color manual and small electric dryer.
If I remember right Jon Weiss has an auto in there