2 things - when they are asking you the questions at the border, they are more interested in your reaction than your response. They are looking for nervous people and will then wonder why you are nervous. So stay calm!
DH and I got stopped at border once and car was searched. As we sat in the waiting room and looked around, I noticed all the men who were being searched had facial hair. Profiling? scruffy = criminal?? Who knows?
They said the reason we were searched, we had been through some drug areas, like Texas. Like the drug fairies had put drugs in our car when we weren't looking, lol!!
We were driving a little old VW car, someone driving a big RV who had the same itinerary would not have been searched, IMHO.
I guess it doesn't mean anything to them that you have gone all 60 years of your life with no arrest record. Everyone is a suspect.