July 5 is a Sunday, so traffic should be fairly light (unlike the other side of I-15, with thousands of cars returning to Southern California).
When you leave
Disneyland, take Ball Road (the street above DLR) or Katella Ave. (the street between DCA and the Anaheim Convention Center) to the 57 north to the 210 east to the 15 north. Stay on the 15 until you get to Las Vegas. If you don't hit any traffic and if you don't stop at all, the drive should take about 4 hours; however, my DH's best time is 3 hours, 45 minutes from Anaheim Stadium to our house. It is an interstate all the way, and yes, you are in the middle of the Mojave Desert for about 3/4 of the trip. This is the kind of driving that you need a AAA card for if you break down. The only "desolate" part is between Barstow and the Nevada State Line; you truly are in the desert with gas stations & services few & far between. Before you get to Barstow, you leave the LA Basin and drive through what is called the "high desert" (Victorville, San Bernadino, etc).
It's always a good idea to have water with you in case you break down or get stuck in traffic. The worst return trip to Las Vegas my family has ever had took 7 hours. We left the Paradise Pier hotel at 4 pm and got home at 11 pm. There was an accident involving two tour buses in the south-bound lanes of the 15 after we passed through Baker. Helicopters, fire trucks, ambulances - the works. Traffic on our side of the 15 was REALLY slow, even tho the accident was on the other side, from all the rubberneckers. I wouldn't wish this on my worst enemy.
Fill up your car before you leave Anaheim; we usually have about quarter of a tank when we get home. Gas stations in the middle of the desert (Baker) charge whatever they want, usually a dollar more per gallon than in the LA area. Last year, the day we came home from our Disney Magic cruise, gas was $4.99/gallon in Baker. No joke.
If you have to stop for a bathroom break or dinner break, I suggest the Lenwood exit in Barstow. This is about an hour and a half from DLR. There are a few truck stops, two outlet malls and almost every fast food outlet you can think of.
There is some construction between Baker and the Nevada state line; they are adding lanes to the 15. Just watch for areas that have a lower speed limit.
Have fun planning your trip!
Any more questions, just ask!!
We will be in DLR 7/5-7/8. We are planning on driving to Vegas that afternoon. What is the drive like? How long does it take? Is it desolate desert roads or interstate? Do I have to bring water with in case of car trouble? 
We've never been to Vegas OR DLR so we are confused!
