1) never let the water go below the plates
2) when done camping make sure they are fully charged before storing the trailer. Trojan's need at least 14.8V to get to a full charge
3) once or twice/year equalize them. that is harder to do, you need to hit each battery till they get to 7.75 or preferably 8V and SG is near 1.280
that takes a special charger
4) don't worry about deep discharging them if you can recharge in a few days. They are spec'd to last over 500 cycles discharge to 25 percent, don't worry about the don't go below 50 percent
do all the above and they will last a long time. I have one set that just now is needing replacement. Batteries were installed in 2004 and had seen 25 to 50 discharges down to 25-30 percent capacity each year.
Another set with similar use is 7 years old and they are barely broken in.