You're gonna want more of a 'dual sport' tire over a full knobby for normal dirt/gravel roads. On my KLR on dirt roads my Avon Gripsters were alot more comfortable than my Dunlop D606's. Once I hit mud and real off-roading, the gripsters become useless.
You're gonna have a hard time finding a dual sport tire for a 15 inch rim, any factory D/S come with larger back rims.
I don't have any experience with it, but if you have spoked rims you may be able to lace on a bigger rim.
Good luck! Obviously a dual sport would be easier for the trip, but ride what you like and make the adventure your own!
Motorcycles are not a form of expression, if you want to express yourself, write a poem, if you want to ride, get a motorcycle.
1986 Honda Shadow VT1100 (sold)
1957 BSA Bantam D3 Major restoration project
1981 Honda CB750F
2008 Kawasaki KLR650
2011 Triumph Sprint GT
2006 Kawasaki ZX14 (stolen)
2006 Yamaha FZ6 (sold)
1999 Honda Shadow Aero 1100 (sold)
1980 Yamaha XT500 (sold)