Ok, this seems like a silly question, but I will give it a try.
In the US, if your not on the interstate system, then you likely won't have this problem. There are few "traveled" roads, that don't have small towns and what not along the path. They do exist, specially in the rocky mountain regions. Adjust the answer as needed.
Pull off an exit, pull back on the entrance ramp. As you go up the entrance ramp pull way off the ramp (your more likely to be seen if your on the entrance ramp then the exit ramp, also cars are generally going slower). Open the Passenger door. Stand at the passenger door facing the inside rear of the car.
Number 1 - Male
Whip it out. Point and shoot.
Number 1 - Female
Uncover. Let it flow.
Don't. Just don't. Wait till the next stop. Even if the next stop is a ways away, you can hold your number 2 for a while (normally). There's generally no reason you can't make it to the next stop. (remember that on an interstate the furthest point between two rest areas is about 2 hours)
Can't stop - Won't stop
My grandfather was a professional truck driver. He always said, don't do the can't stop/won't stop method. There is no reason to risk your health just to save a few moments time. When you have to go, go. That said here are your answers.
Don't be stupid. Driving with your wang out is a recipe for disaster (I don't imagen it's much different for gals. Doesn't really matter the circumstances. So lets assume your not driving.
You need a container that is large enough to hold all the urine. You will need a container that can hold a liter or more likely (for adults). You don't want to have to switch containers.
Number 1 - Male
If possible you want the container to be large enough to (sorry) "put the tip in" but that may not be possible. Your basically working with what you got. Like before point and shoot. If you can find a bottle with a large enough opening, rest "it" just inside the top, if not then line up the "entry and exit" holes. Just make sure that you have a large enough container.
Number 1 - Female
Ladies have it harder on this front for sure. First you need a funnel. A real funnel may be nice (if not coated in engine stuff, remember where your about to stick this thing). If that's not an option time to make a funnel. Cut off one corner of a large bag of chips. A plastic bag, will work as well. Cut one bottle in half and just it as a funnel into a second bottle. Just make sure the opening on the receiving bottle is larger. Scoot to the edge of the seat. Cover up as much of the "area" as you can with the funnel. Let it flow.
Nope, just don't.
Super Best answer
Pay more attention to your body. Pee when you can, poop when you must. Every time you stop pee. Every time. Bring an ass gasket if that's your concern. Every time you get gas (300 miles is only 5 hours. That's a stretch for kids, easy for adults). If your traveling with kids plan on stopping every hour. If your Traveling with just you, or just adults, plan on stopping ever three. Do a pee check at every stop. "Were not going anywhere till you pee mister. Now get in there and pee!" if you have to. A reasonable amount of planning is really all that's required. Again if your in the US and on the interstate it's rather difficult to get caught if you stop at every rest area and make sure everyone pees (again assuming everyone is healthy). If your taking the highways, then there will almost certainly be a small town, though there are a few areas that don't.