I used the same Blue Sea ON-OFF switch as the one in Teamfour's link. I mounted it on the top front face of the battery box facing the propane tanks so it's protected from driving rain while traveling. Plus, mounting it on the front vertical face and immediately below the lid ensures better protection and water run-off than mounting it on the top of the battery box.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)