If the switch by the sink has a light built into it, it is likely the on/of for the hot water heater.
When you turn it on, the light comes on for a few seconds and then goes off. The heater should then light within 15-30 seconds. If it doesn't the light comes back on and stays on. If you recently changed propane tanks, it may take a few cycles of this to purge air from the lines.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
Amateur Radio kd2iat monitoring 146.52