Driving on the highway in the States is generally very straightforward. Have your husband drive in the city and trade him for highway driving, if you're nervous. You will be surprised by how big and spread out everything is. Even in the fairly busy east coast, people don't think very much of going for a 1-2 hour drive for a particular/to see a friend/to commute/just because. Gas is stupidly cheap, the highways are wide, (almost) everyone has an automatic, and people spend a ton of time in their cars.
In many places you're going there won't be more than three or four cars a mile (depending where you are, of course); you can just set the cruise control to to the the posted speed limit and just steer the car occasionally.
Remember that everything is backwards--so the slowest lane is on the right. If you sit in the right lane going 64-66 in a 65 zone, you will almost certainly be entirely fine.
Very few roads in America have speed cameras, so if there's speeding, it will be a trooper pulling you over. As they can only pull over one car at a time, they tend not to waste time on anyone going roughly the limit--because while they have you pulled over, some yahoo will fly by at 87 and they've missed their big ticket.
If you're worried about driving while black, there's plenty of advice on the internet. My note would be to keep your hands on the steering wheel if an officer approaches the car; my guess is that your accent would cancel out any racist preconceptions they might have, but it's something that I tell all my friends (of all races). I am not undermining the seriousness of your concern, because it is real and fair, but I will tell you that chances of something bad happening in this regard during your vacation are slim.
As for Death Valley, I would personally avoid the gross biker motel and stay at somewhere kitschy and fun. Stovepipe Wells has fun reviews--or you could splurge a bit and go for the Inn at Death Valley. Spend an extra $40-60 for something you'll enjoy. And make sure to gas up your car whenever you can!