Rob
I have never travelled on an American Coach but I have had DVT about 10 years ago. You will need to stand up and walk frequently, not just sit back and stretch your legs out. In the UK it is illegal to walk up and down coaches whilst they are moving, in case of an accident, but I don't know the law for the USA. It is not a problem in the air. Just ask the steward/stewardess especially if there is only one aisle. Do not forget to wear the pressure stockings.
The most important thing of course is to make sure that your insurance company knows about your condition because some of them will not entertain you.
I hope that you decide to do your holiday, and I am sorry if I have been so negative.
Best wishes
Peter