Dog park stops!

We moved from DC to OK a few months ago and we took our pups to a dog park they love in DC right before we left. They ran around for about an hour, then they slept most of the way. Everytime we stopped for gas, they would get some walking around time. If there was an open area, they would be let off-leash.
We stopped outside of Nashville for the night and in the morning took them to an excellent park there for an hour or so. They slept most of the morning.

As a former MN resident, I'd say make sure to buy a lot of doggy sweaters for the little gal. :)

That's a pretty decent haul, are you doing the driving all in one day or two? We used to live in Minnesota... a coat is needed for sure, or sometimes double sweaters can actaully retain heat better.
Depending on where you will be walking them, boots, or those little rubber "boots" (they're kinda like rubber gloves for your dogs feet) can help. The slushy parts of snow and ice that is found on sidewalks, alleys and streets freezes between their paws and from past experience, that is game over for your walk.

There are great dog parks in Minnesota! There are also great parks in Kansas City Missouri & Kansas which would be a natural half way stopping point. If it's your first time with a really long car ride, sometimes some doggie dramamine doesn't hurt. They will have to pee each time you stop for gas for sure, but it does knock them out the entire way if your Boston gets nervous about trips longer than an hour.

Make sure to carry a bowl and a few water bottles with you, they will drink more than they will eat usually on really long drives!

When I moved to the UK I got Cole a nice winter jacket :-)

BOOTIES! those lil rubber ones that look like balloons seem to work best.