If you're going to continue draining the tank after each trip you'll probably have years of trouble free use from this one and have a good handle on the deterioration. If you have an RV store handy I'd hold off buying a spare until you really need one and save losing or damaging it before you even get to use it. If you don't have a store handy then for sure get one so you're ready when the inevitable happens.
2007 Shamrock F19 hybrid - 2010 (22 nights), 2011 (24 nights), 2012 (11 nights)
2013 Surveyor Select 235RKS - 2012 (6 nights), 2013 (32 nights), 2014 (26 nights), 2015 (37 nights), 2016 (14 nights)