Before you begin, have new mixed saltwater in the new place, just waiting. Make sure SG matches with the original tank water. Make up enough refill the tank, and also to replace what you waste when you acclimate your critters.
Sand is heavy...it will stay plenty wet for just 20 min ride...so just put it in a bucket, there will be enough water already in it and you will even see it on top.
Rocks, I put mine in a new plastic garbage can for a 2 hour ride...it will not dry out, so no need to put water on them.
Put coral in a tub filled just enough to cover the coral, that way splashing water will not get in your car....keep the coral in the dark, this will prevent damage and stinging each other... Same with the fish in a different tub. Use a heater in each tub with an air stone or small power head to circulate water.
Reset the tank, add only half water from the new mix....add rock FIRST, then sand, place your power heads where you want them (don't turn on yet). set up filtration...refill the tank using all the water you saved, top it to the desired level with the new if necessary. Start the filters and power heads...let everything settle.
Acclimate the corals and fish, just like when you first got them. This is why you need so much new mixed saltwater...in the end it should be like a huge water change, so no spikes.