Re: car sick cats?

Have you had them checked out by a vet?

Are they in carriers while you're on the road? If not, is it possible they are getting into something when they first get in the RV?

Have you changed their food?

I would look at things that have changed other than just their age and get the checked out. Seems to me, it's more likely to be something else that is causing the trouble if they've traveled in the past without issue.