I have to admit I wouldn't take my Enfield on such a trip. If it weren't for my disability, I would have no problem taking a trip like that on my Yamaha XT225. Engine is less than half the size, but power is about the same. It has an aftermarket center stand and a 4.5 gal. gas tank on it. It has never broken down no matter what I put it through. It has over 30,000 miles on it, more than half of them off road and on dirt roads. Still runs like new. But I am reaching the point where fixing a flat beside the road or out in the middle of the desert is a bit more than I can deal with. It took me more than 4 hours to change my last off road flat, and I was so exhausted I almost wasn't able to ride back out.
I think way to much is made of engine size. I had a 234cc Honda Rebel that I put over 10,000 freeway miles on, at full throttle. The only real issue is that the bike was to small for someone my size. (the bike, not the engine) I would have no problems with a Yamaha V-Star 250 or Suzuki TU250. I have ridden all over the U.S. on Japanese 400-450 size bikes. One exception seems to be late model 800-950 Japanese v-twins. Despite the engines size, they make very little power. Supposedly it would be about the same riding long distance on a Yamaha V-Star 950 as it would be on a V-Star 250 powerwise. You could pack more on the larger bike. My old well worn Vulcan 750 will run circles around Japanese 1500cc v-twin cruisers.