I remember my dad saying something about this when they would head south for the winter. He said that he just showed the clerk his CDL and they gave his the discount, since he has kept CDL current after he retired. Probably not much help since most people don't have CDL's, but if you have a CDL you also will have a med card...
TT-2013 Passport 3220BH
TV-2004 F150 FX4, not exactly stock...
with 2001 Kodiak K215: 2010-10, 2011-12
with 2012 Grey Wolf 26BH: 2012-19, 2013-24, 2014-11, 2015-6
with 2013 Passport: 2015-13, 2016-15 booked