Which State to Claim?

While on a recent trip to CA we bought a new RV. We are Colorado residents and planned to register the RV in CO. However, we met a couple of RVer's that held SD licenses and suggested we do the same. We have since been trying to find out what constitutes a CO resident, not very clear. For the last 2+ years, we currently travel very close to 6 mo of the yr. We still own a home in CO and typically come back to see grandkids, doctors, dentists and to take care of the house and mow the grass. We generally don't spend more than a month at home before we are on the road again. To register our cars and obtain driver's licenses in SD would save us quite a bit of money each year. Our only concern is since we are not full timers, could we legally claim SD as our residence under one of the SD mailbox companies. What type of pitfalls could we encounter. Thank you ahead of time for your responses.