Afraid that I'd definitely say no sedation on a long car journey. They're better just their normal selves - well ...... grumpy, but normal.
I've travelled extensively with cats - both touring and on long one-off journeys. It's not their favourite thing but they manage pretty well.
I'd recommend a good sturdy, roomy carrier with no jaggy bits (like some of those wicker boxes people use) and the floor covered with a rough folded big soft towel such as a bath towel. Best to put the box in the back/on backseat, anchored by seat belts or some secure way in case of emergency stops.
They usually don't want to drink on journeys but it's as well to have a container of water and a plastic bowl or two in case you want to try them with some at rest stops. (They almost certainly won't eat so I wouldn't bother with food.)
Also take a spare towel, some kitchen paper towel, cleaning spray and poly bags - in case of accidents (I've only ever known one because they usually cross their legs.) Having a litter tray around is good. Even if they don't want to use it at pit stops, it's ready and waiting to be taken quickly into the destination house and put into a quiet room for them to use on arrival.
I'd recommend part covering the box with a light rug but leaving a crack so that they can see you in the car. They'll generally start with a protest then just hunker down at the back of their carrier but giving the occasional squawk so that you know they're there. Just answer them briefly, maybe a quick finger stroke through the bars if you can reach them, and then ignore them till the next squawk.
Finally, (I think), I really wouldn't recommend that, however well behaved you think they are, you let them out of their carriers at all unless the car is in a pit stop with all the doors and windows closed. A loose cat in a moving car is only fine as long as everything else is fine. If something goes wrong on the road or in the event of a small accident, they're a menace. My own boys have usually just been left in the carrier until the destination is reached - they start to feel safe there.
Sorry for the tome. It sounds worse than it really is.