We just got back Saturday from a long trip (13 - 15 hrs each way, depending on traffic). We drove straight through also, with only potty breaks. We even ate in the car. To start off with, our kids are thankfully very good travelers. Little to no whining, and no crying. My younger two are 7 and 4. The older boys (16 and 21 now) have pretty much been on their own for entertaining themselves for a long time, so I wouldn't have even worried about your teenagers if it were me. They have books, video games, and iPods to keep them busy. For the younger ones, we watched movies some of the time. We have a portable DVD player that only comes out for long trips. I don't care how many movies they watch when I'm trying to keep them occupied on a long trip, though. :) They both have a Nintendo DS, so they played them some. I gave them a couple of new games to try out, which caught their attention for a while. I also brought along their V-Smiles with all of those games, but they never got them out. We had coloring books and activity / puzzle books, and I surprised them with the Bendaroos they had been asking for. That kept them busy for a long time. My 7 yr old did all of her make-up homework on the car ride also, which took a couple of hours. Bring books to read, or just look at for the 3 yr old. We also play a lot of car games, like the alphabet game, the state game, and yellow car (a made-up game my son's girlfriend taught them... you keep track of who sees the most yellow cars first. When they made it up, yellow cars were scarce. Not so much now, though. We are thinking of changing it to orange car since they are more difficult to find. The little kids seem to like this game the most, and the older kids play along with them.) Make a trip to the dollar store and stock up on things for them to do. That's where we got some of the activity books and coloring activities. I think part of the reason my kids were so good is because they were so excited about going to Disney on the way down, and they were so excited to get back home on the way back. :) They had something to look forward to both directions. Also, they don't often get new things when it's not a birthday or Christmas gift (or something they had to buy themselves), so the surprise gifts made them very happy and content.
I hope you have a great trip!