Jeff,
In my experience including Georgetown, The Valley RR, Cumbres & Toltec, White Pass & Yukon and the RR Museum in Golden we never had a problem with condensation. We used both bagged dry "play" sand and sand we dried ourselves. Leaks in the side of the dome from rust and in the lids did cause problems. None of our domes had gasket seals. At Roaring Camp we have experienced exactly the problem you are referring to. Venting the dome lid did not work. The solution was to put a lighted 60 watt incandescent bulb in the dome at night. That solved the problem.
John Bush