I'm not talking about using gel batteries as cranking batteries, I'm talking about using gel batteries as a house bank, just the way you are using them. If they aren't properly charged, and monitored with a battery monitored (and a voltmeter doesn't even approach "properly"), then instead of lasting 8-10 years they're likely to last 2-4 years.
As for maintaining a proper voltage, there's only two ways to find that out for sure. One is to measure the specific gravity, which you cannot do with gel cells. The other is to use a battery monitor. A third but less precise method is fully charge the batteries, then disconnect the batteries from any charging source and loads, and let them sit at rest for 24 hours, then measure the voltage. It is quite impossible to properly charge a gel battery with an automotive or truck alternator unless the vehicle's charging system is set up for gel batteries, and none of them are. You haven't had any problems yet, so you think there are no problems. There's an old saying, "Batteries don't die, their owners kill them," and usually without knowing why. Now, at least, you'll know why when your batteries die.
"You should never use a gel cell battery for engine starting."
Why not? It won't harm the batteries in any way, and as long as the batteries have enough cranking amps, they'll start the engine. Gel batteries are often the primary starting batteries on many boats, especially larger sailboats with engines (yachts) and houseboats. It wouldn't be smart to stick a gel battery under the hood of a car or truck to use as the primary cranking and vehicle battery, not because it shouldn't be used to start the engine, but because the vehicle's charging system cannot properly charge a gel battery. Not unless you were to replace the vehicle's alternator with something like a Balmar alternator and regulated charging system specifically designed for gel batteries.
I am curious to find out the difference between "deep cycle" batteries and "deep deep cycle" batteries. A "deep cycle" battery is designed to be discharged to anywhere from 50% down to 20% of charge. Are your particular batteries designed to be discharged deeper than 20% of charge? Although deep cycle batteries can be cycled down to 20% charge (80% DoD - Depth of Discharge), the best lifespan vs cost method is to keep the average cycle at about 50% discharge, as there is a direct correlation between depth of discharge on the battery and the number of charge and discharge cycles it can perform.
While gel batteries can handle deeper discharges such as 80% DoD better than wet cell and AGM batteries due to gel batteries having 1.5-1.7 times more electrolyte than AGMs and wet cell batteries, and they initially have more cycles per the life of a battery than wet and AGM (at 50% DoD gel batteries have approximately 1000 cycles of life, whereas AGM batteries will have about 370, and at 80% DoD gels have 600 while AGMs have about 200), the correlation between deeper discharges and charging cycles nevertheless remains true regardless of battery construction.
Here's a data sheet on gel batteries that you should probably familiarize yourself with, paying close attentions to just how important temperature is to charging voltage. You'll see that your 14.1 volt charging system is probably overcharging your batteries, depending on where you have your batteries mounted. You should also pay close attention to the table in the Open Circuit Voltage section, because if your batteries show a stead-state of charge at 13.2 volts for even a few hours after shutting off the truck, much less for days, then there is something very, very wrong with your voltmeter.
http://www.bethandevans.com/pdf/Batterydata.pdf
