Well, at five years it's probably on its last legs anyway. 5-7 years is the usual life span of a good quality deep cycle battery. Lesser ones may last only 3-5 years. So yes, replacing it is a good first step.
But boiling over is not a sign of a good charging system and I wonder if you may have a defective or perhaps just a mediocre-design charging system. Or perhaps you have been allowing the water level in the batteries to get and stay too low. You didn't mention the make and model of RV or charger, but older Magnetek converter/chargers were notorious for boiling away battery electrolyte ("water"), so if you have one of those it might be money saving in the long run to replace that as well. The new chargers from Iota or Progressive Dynamics (with Charge Wizard) will insure a long and helthy battery life.
Like Gary said, probably a bad battery. Do all the cells boil or just 4 or 5 of them? If so, that would indicate one or more shorted cells. If the charger (converter) is at fault, the battery was probably ruined by it anyway. Get a new one.
Gel cells have their uses, but typically not for long term, rv deep cycle use. Either a flooded cell deep cycle or AGM deep cycle battery is better. The choice seems to be equally split between the two - the AGM's are, as Ned stated, no maintenance, but are considerably more expensive than an equivalent flooded cell battery. You need to do a cost/benefit analysis to see which is best for you.
Gel cells are good for uses where they can be any way up but straight, Updside down, on their side, and so on.
The trade off is they can not, as a rule, deliver a whole lot of current, I mean 40 amps is a whole lot of current to a gel cell.
AGM's on the other hand are also sealed batteries, however they are designed to operate upright only. not on their side or upside down. Now a good size AGM can deliver 400 (or more) amps, no sweat.
Flooded wet cells come in a variety of flavors but the true DEEP cycle types are similar to AGM when it comes to current (Slightly higher) and power (Amp Hours per pound) since the chemistery is the same. They have to be mounted right side up and you need to monitor them from time to time to insure they don't boil dry. But all around they give the most AH per Pound and they give way more AH per DOLLAR. And that is important.
The only disadvantage is that they need to be in a vented area and you have to monitor them and clean them from time to time.
But overall. that is the argument for them.
I have three battery banks on my coach, The Chassis battery (Maintance free, I have no clue what is inside the case)
House main, (Flooded wet cells, one pair of Interstate Work-A-Holic U-220 Golf Cart Batteries) and House-Inverter (AGM) The AGM's are in a place where venting is a problem (The compartment is vented, Power vented if it gets hot enough) I've tested one of the AGM's at about 450 amps for 15 seconds and it passed 450 amps is the most my load testor will draw If memory serves 450 amps is the test rate for a 150AH battery, it passed with flying colors (There are two AGM's the other one did not do as well)
John, your're wrong. AGM batteries can be installed and operated in any position. The capacity of a battery is not dependent on it's type, but on it's design and size. AGM, gel cell, and flooded batteries are available in many different capacities.