As Scalf77 said, if in fact you have a gel battery, they require a different charge profile than other types, typically 14.0v - 14.2 (constant voltage charge) and float 13.1-13.3 and are known to be sensitive ie. life limiting to overcharging due to gassing....Your alternator is typically a bit higher (14.3-14.5) in terms of output and long drives...are they really gel?
Here is a nice summary table comparing characteristics of lead acid batteries
Does your built in charger have charge profile settings? (my Fresno build did not) You have a starter battery in parallel typically that wants a higher charge/voltage that will be supplied by the alternator, so that is a consideration....
You could get sophisticated and go from alternator to inverter to a charger with a gel profile.....There is a nice write up on that here.
But, as you indicated, the price is right...