[High5er]

You mentioned you're full timers. We full time as well and leave our gray valves open when the overnight temps are above freezing (I close them periodically to allow them to fill and drain to minimize the nastiness inside them and the hose to the ground). I found that if left open, the draining black can create a vacuum that empties the P traps in your sink/shower drains. The result is wide open pipes to the sewer/septic and those gasses are free to flow inside. If you're leave the gray open most of the time, I would start closing it every time you drain your black tank.