I don't know about the particular truck you will have, but we've had multiple p/u trucks. Two car seats in the front seat next to you? Squished! (I would imagine.) If there's a bench behind you, I'd put the front facing behind you so I could move up the passenger seat to accomodate the rear-facing.
I did a three day trip with my 7yo, 3yo, and 2yo. My parents encouraged me to get up and out early, so we could get to the hotel destination early enough to enjoy the pool. Me:
My plan was instead to drive the most when they napped the most. So, since mine are most active in the morning, we would leave after an early breakfast, drive for about 2 hrs, and stop for an activity for a few hours. Think of something small, hands-on and free/cheap - we visited a small butterfly house, the John Deer Museum, and something else... We'd grab a small lunch and be off for a long drive, while they napped away the afternoon. I tried to find a good hotel with a restaurant right next door. We'd park the car, go check in the hotel, then walk to the restaurant (need to get the wiggles out before sitting again for food!). We'd then take a longer walk back to the hotel, and try to get more energy out before unwinding in the tub, and snuggling in bed. Never ever did I consider finding time to hop in the pool - I am not an octopus.
We had a WONDERFUL trip. I ordered AAA tour books ahead, and highlighted things on our route, so I could be prepared for a needed stop.
I had ONE bag that we took into the hotel (in addition to the diaper bag). And I had my sling for the little one. I kept snacks in the car for the ride. That bag didn't leave the front seat.
I realized that I had to be flexible. I could either make it an enjoyable adventure, or we could all suffer a horrible trip. Fun music and stories on tape/CD will help too. I did not make hotel reservations ahead, since I wasn't sure where we were going to be able to stop. One night, where I thought we would stop turned out to be a dump. So we got back in the car and drove further. I knew there was a possibility we might need to take an extra day. If we needed to, we would have.
Good luck - and try to enjoy the adventure!