With its proximity to the city water versus tank switch, I would tend to lean towards it being a second water pump switch. In the Legacy, there's a water pump switch on the water control panel as well as one inside the coach with all the other controls. I would expect tank heaters inside so you could turn them on if needed without going outside. If you flip it on, do you year the pump pressurize?
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!