Usually fouling is attributed to a too rich mixture. Using convenience store gas doesn't help. Since the Rock is a low compression engine any contaminants in low octane fuel will plate out on the electrode. This causes a slight loss of spark efficiency and repeats several thousand times a minute riding down the road. The build up of carbon continues until enough efficiency is lost that the current flow in the spark cycle goes to ground without any spark at all, thus no fire in that cylinder.
Usually the way most spark plug manufacturers combat the fouling issue is to reduce the size of the electrode and ground. Smaller diameter wires limit the carbon build-up. Some people go to a hotter plug which is more effective at burning off the contaminants, however there is a risk with going too hot and burning holes in the piston crown.
I recommend staying in the same heat range (or one heat range higher) and going with a smaller electrode. About six months after I bought my Rocket (less than a thousand miles) I replaced the plugs with Iridium plugs from NewR3.com (I just wanted a better plug than OEM). Now 5 years later those plugs are still in there and going strong. I also try to only use name brand gas (Phillips 66, Amaco, BP, etc) since they tend to remove more contaminants for their stations than what they will sell to 7-11 and Love's Country Store.
Plugs are cheap enough to carry a few spares. During your trip you can always pull a pair, clean them and re-install. I replace the plugs in my tractor before every use. I have enough plugs to pull and replace, then clean the ones removed for the next time. Course the engine was built in 1941 and cylinders 1 and 2 tend to oil foul the plugs pretty bad due to valve seal leakage but that's another issue.
Good Luck
