Car space

So far our Ford Taurus has been a good purchase for us. We bought it right after Lexi was born last year because it was very evident that our little 2 door Hyundai Accent was not going to be big enough to haul around all the stuff needed to take a baby on a trip. After all, we could barely fit the car seat in the back seat.
This last camping trip we had a lot of stuff and we didn't even bring a tent, stove or any other camping gear. We had a cooler and some food, as well as our luggage, small stroller and bedding.
We barely squeezed all our stuff in the empty space (which included the trunk and the back seat next to Lexi.)
This trip really made us realize there's no way we could travel in this with 2 children. Unless we strapped some things to the roof. We thought about getting a cargo bag for on top, but last year on our trip to Maine with my mom we learned those leak really bad in the rain unless you have a roof rack on the top of the car. (It's really not fun riding from TN to Maine getting wet. My step dad got so wet he looked like he rode on the roof).
I don't think you can just put a roof rack on your car like you can put a truck rack on your truck can you? I think it's one of those things that needs to be put on when the vehicles is made.
So maybe we could purchase a trailer of some sort to avoid the cost of purchasing a mini van or maybe a semi truck.
cccccccccccxxxxxxxxxxxxxxxxxxx (That's Lexi's input)
I was the youngest of 5 and I remember riding to the campground sitting on my sleeping bag and my legs propped up on coolers (and we had a huge van). So how do you travel? Do you have problems fitting everything in the car?