Hello, and welcome.
It would be best to discuss this first, with your vet, who knows each of your cats' medical history the best. The dosage would depend on each cat's weight, age, and tolerance of this class of medication.
This is a long, cross-country trip and you cannot keep the cats sedated for the entire time. I don't know if you plan on making stops along the way to sleep in motels, etc., but, legally, we are not permitted to tell you a dosage for a controlled substance medication, since only your own vet can do that; however, you can find general dosage, precaution and side effect information about diazepam at this site:
Here are some great tips for travelling cross-country with your kitties:
I wish you much good luck on your move and hope the trip goes smoothly. If you need any additional information, please let me know by clicking 'Reply.'
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