Our family hasn't been on any long vacations in over a year (mostly because my wife became pregnant with our third last year). Now I am trying to convince my wife to go on one which entails a 7 hour road trip. She is reluctant because our 8 month old is a bit under weight, and she has been getting sick quite frequently in the past few months (mostly with colds, nothing major). She thinks the road trip will make the baby weaker and more prone to getting sick again. Is she right?
This is from a recent experience, not a pediatrician advice.
I have a 8.5 month old daughter, underweight. She had a slight cold at the time but she's usually pretty though (my wife caught a minor pneumonia while our daughter coughed twice...).
We did a 6 hours trip one week ago. Both for our kid and us, we took a stop every 2 hours (it's a common recommendation for long car trip anyway). It drove the trips to 8 hours, but everything went smoothly. The first and third stops were for meals. The first one was for everyone and the third was just for our daughter afternoon meal. The second stop was just moving around, we got the kid out of the car and had her play a bit with us. The return trip was more or less the same with a slight variation : our kid ate before leaving, first stop was to move around, second stop was her afternoon meal, third stop was for our diner, but she sat with us and played a bit. In both case we gave her her bottle of milk when we arrived. As she is quite calm, she didn't complain even if it was late, so you might have to include a last timed stop to feed her.
Both trips went fine, she got back home better than she left. Keep your kid warm in the car (but not too much), give her toys adapted to her age. If you feel it might help keep a handkerchief with some menthol near her cosy to help with colds and such.