Oh boy. We just got back from a 2000 mile trip with the 10 month old (and the five year old). It was okay, but there's no way we could have gone that far when she was five months old. She has always been a child who wants to be upright, hates being buckled into anything and doesn't take a pacifier so we kind of had our troubles set for us. The only way we could make this trip is by getting a convertible carseat and Baby Einstein dvds for the car. We also split the trip up into 8 hour days, stopping usually every four hours or so and leaving each time around naptime so that she'd sleep for a little while at least. We were really flexible and tried to be patient. There were times when I had to breastfeed her while we were driving down the road. NOT FUN. But we made it and she did too and it wasn't too traumatizing. I think it would have been hard when she was five months old though because she was much less distractible - didn't care about toys so much and wasn't eating any solids.
FWIW, my other dd is a traveling champion. She rode with me cross-country monthly from when she was 6 weeks old and was awesome. I had to be patient of course and not try to drive for too long at a time, but she really just hung in there. She took a binky though and is generally not as intense of a child. I really think it has to do with the kid and their temperament.
Mama to Maia (12/04), Nora (9/09), Sam (8/12) and Step-mama to Aidan (3/02) and Luci (10/04).