We puzzled it out in another thread. It's not really a function button; it's a tank level status light button like for fresh, black, gray, etc., for a tank for a washer. However, the level status panel itself includes the statement "TANKS MAY VARY BY MODEL".
SEARCH is your friend!
Nights Camped: '13 = 49/'14 = 74/'15 = 74/'16 = 85/'17 = 110/'18 = 111/'19 = 86/'20 = 26; booked = 42
FR Wildcat 295RSX / GMC Sierra