The "normal" lifespan of most batteries can be determined by the warranty period; battery warranties are figured in the favor of battery manufactures. Let's say you buy a 60-month warranty battery and it lives 41 months. The warranty is pro-rated so when taking the months used against the full retail price of the battery you end up paying about the same money as if you purchased the battery at the sale price. This makes the mnfr happy. The battery mnfrs spend millions on research and have a very good idea as to when a battery will fail under normal usage.
Our goal is to exceed the mnfrs "lifespan" of a battery. The good news is that it's very easy to do provided that you properly maintain the battery. For starters, I suggest that you employ a multistage battery charger (aka smart chargers). Was your trickle charger putting out a constant amp charge? Second, I recommend that you get a hydrometer, where you measure the amount of sulfuric acid in the electrolyte for each cell (which is more accurate than using a voltmeter on the terminals). This second step ensures that you're also checking the electrolyte level.
There are many resources via the web and/or via forums such as this one regarding battery maintenance. See, for example, http://www.marxrv.com/12volt/12volt.htm
and commercial vendors such as www.batteryminders.com
I use a BatteryMinder smart charger to maintain my 6V golf cart batteries and another BatteryMinder charger to maintain the battery in my tow vehicle when not in use. Some folks have a smart charger incorporated in their converter (and I recommend you replace the OEM converter with a smart-charging converter, depending upon how you use your TT) and simply remain plugged-into shorepower (eg, during storage). Others may suggest a solar charger. Either way, the goal is to properly maintain the batteries for extended use. Sorry to hear about your battery; I hope your new battery will last several years.