The last longer road trip was from N Florida to Newfound Land island. Took two mountain bikes and two road bikes. The mountain bikes were on a rack behind a small travel trailer.
Made it a point to ride every two or three days at least. Otherwise you lose condition fast if you are at an advanced age.
Sold the trailer, next trip is too Moab and surroundings with a Honda CRV. Two mountain bike will be on a receiver mounted rack.
I don't worry about scratches or anything else except water in the bearings. It appears that doesn't happen often
I lock the rack to the receiver, lock the bikes to the rack including the rear wheels. I use a very thick cable and a strong lock. The front wheels are in the vehicle. I also use bike bras to keep the sun off the leather saddles and under the bras are vinyl-cloth covers which the wife made to keep the rain/dirt out of the handlebar stuff.
Al