Late April shouldn't be a problem, as the highest point on either route would be under 8000'. This winter there has hardly been any snow on the ground along either highway, although with this crazy weather, who knows what can happen! We had 3 inches of snow once on Memorial Day weekend! Your biggest problem may be finding RV parks that are open that early in the season. Santa Fe to Mesa Verde is an easy one-day trip, and there are a couple of year-round RV parks near the National Park, so you should be okay. Most of the local RV parks (Pagosa-Durango-Cortez) open in early May. We like Santa Fe Skies RV Resort in Santa Fe, by the way, and I think they are open all year. Easy access off the loop highway, great views and a nice facility.