I would look for a compact one piece residential toilet. It should work great and some have dual flush capability (1.1 gal/flush), or the standards are 1.28 gal/flush. With one piece you don't have to worry about the tank bouncing around. And if you turn off the water supply before you begin your travels, flush the toilet, then you will be traveling with an empty tank. No worry about sloshing.
Something like this: http://www.americanstandard-us.com/toilets/Compact-Cadet-3-FloWise-One-Piece-1.28gpf-Toilet-2070/
I've seen them cheaper at the big box stores.
I think you could make it work.
