Stevenp,
I used to live in Minnesota, but moved to Vegas over 6 years ago. I have to tell you...despite the crappy winters MN is one of the best places I've ever lived and can't wait to someday go back! You didn't say how long your trip was going to be or when you were going to leave, but at that time of the year, I'd spend as much time in the northern part of the country as possible and make my way south in the fall. I ride out here in Vegas all the time, and once June comes, my bike goes into hibernation until the temps are out of the 100's in late September. Beyond those few months, riding here the rest of the year is actually very pleasant.
Desert temps in July and Aug are extreme, even for a local Las Vegan. Nighttime temps often hover around 100 degrees in Aug, so even night riding will be extremely hot. Don't forget all the fun little critters that thrive in the desert summers...scorpions, black widow spiders, rattlesnakes...If you plan on any stealth camping make sure you do some homework about where those critters tend to live and how to deal with them if you do come across them. Nothing worse than a black widow bite when you all alone in the middle of the night and the middle of nowhere!
All that said, follow others' advice about hydration, terrain education and advanced planning. There is some really good advice in this thread!
And when you do finally make it to Vegas, I am happy to show you around some of the best riding areas the country has to offer! People only think of the "strip" when they think Vegas, but there is some spectacular riding out on this side of the world!