I'm not trying to scare anyone, but I think we need to be informed about whether its important to have someone sitting in the back for extended trips. It is recommended that the drivers stop every hour and a half if the baby is going to be sleeping in the back because their airway could become constricted when their head rolls forward. The reduced level of oxygen becomes dangerous after awhile, obviously. And this is not the same as saying you need to watch your baby all night in a crib, then, because a baby's head obviously doesn't roll forward if they're placed on their back to sleep. Here are a couple news articles on the subject. HTH.
https://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20070718/sids_babies_070718?s_name=&no_ads=
https://www.thinkbaby.co.uk/news/article/mps/uan/898
Re: Articles on extended periods sleeping in car seats
I always sit in the back when we go on long trips.
Ditto!
"You're gonna miss this You're gonna want this back You're gonna wish these days hadn't gone by so fast..."
Yep that is why when DS had to sleep in the car seat (per pedi's rec) for his gas and reflux issues, I adjust it so it is further back and his head leans back not forward.
ETA: BTW, this kind of flopping forward is the same danger with letting a LO sleep on a boppy and hwy you need to be right there. There was a case a couple years ago here of a 3 mo who suffocated while sleeping on a boppy because he slipped forward so that his head fell against his chest and he couldn't move out of the position.
DD -- 5YO
DS -- 3YO