A.M.
you two are not together. he's not breaking any laws. there is literally ZERO you can do at this point. so your choice is....do nothing, or do nothing. the only choice you have is HOW you react to the situation. you can be the shrew ex wife and cause all kinds of drama (very unlikely to change his mind, btw). you can state your opinion calmly and respectfully (as you state you already have) and then keep your opinion to yourself after that - i.e. handle it with grace and dignity as you should anything involving your ex.
and i'm with Shane. i'm sorry, but travelling 1000 miles to the beach to go on family vacations in the back of the pickup truck (with a camper shell) on mattresses are some of the best memories from my childhood.
understandably you're nervous. but if it's something you can't control (he's not your husband, it's not your trip, you really have no say once you send those kids off with him) there's something to be said for pragmatism. don't make yourself the bad guy in your children's eyes. you won't win this one.
(not for nothing - make sure you are also not being a hypocrite here. because none of us ever take risks while driving right? talking on the cell phone, not coming to a complete stop at a 4 way, not using our blinker...we're all perfect, right?)