Young Samuel, oh, OK, got it. One thing you need to be aware of, and if you're so inclined it's buried deep within the link that Scott101 provided, is that more than likely, the charging system of the truck will different than the charging requirements of those gel batteries. The truck's charging system is designed to charge the truck's batteries, which will have different charging requirements than gels will. Meaning, the alternator will output the correct voltage and current that the truck batteries require, which may or may not be (trust me, it won't be) the same that the gels require. The result will be gels that are charged at the wrong voltage: at too high of a voltage which will kill them quickly, or chronically undercharged which will kill them not quite as quickly as too high a voltage will, but they'll be just as dead.
Gels are super picky about the voltage with respect to the battery temperature, with the voltage requirements being in a very narrow range of acceptability. For example, if it's 25° F outside, the colder truck batteries will require a higher voltage in order to be charged (colder the battery is, the higher the voltage needed), which may be 14.3-14.5 volts.
I'm not sure where you get that the battery combiner will prevent the gels from being overcharged, since a combiner simply links, or combines, all batteries into one. When the gels are connected to the truck's batteries via a battery combiner they will receive from the alternator the same charging voltage and current as the truck system starting batteries. If the temperature of the gels and of the truck batteries are about the same, it's not a problem, but if the truck batteries are 25° and the internal temperature of the gels are 65° inside the bunk, then the gels will be severely fried by the 14.5 or so volts that the much colder batteries will be getting from the alternator.
At 65° gels require a minimum of 13.7 volts to charge at all, and a maximum of 14.1 or they'll fry dead. Just .1 or .2 too much voltage at any given temperature, for any length of time at all, and gels will die a very quick and painful death. Quick, as in meaning, at the top of your Today's To-Do List would be "go buy new batteries".
Like I said, tho, if the truck's batteries and the gels are always nearly the same temperature, within 10 or 15 degrees, then you shouldn't have any problems. Gels make great batteries for boats, since charging systems on boats are easily configurable for multiple charging and regulated voltages. And they're pretty good for campers and others that use isolated, regulated charging systems via generators and shore power. Gels have the huge advantage over AGM batteries in that they can be routinely discharged much deeper without capacity loss, and they have sometimes as many as twice the charge cycle lifespan of AGM's. But that one big drawback for many applications is the critically sensitive voltage requirements they have.
If the truck's system batteries are also gel batteries, then they will all receive the same voltage and current as the bunk batteries, so there's no problem there. But the problem with gel starting batteries is the available CCA of gels falls to nearly zero as temperatures drop below 30° F.
If you have a generator on the truck, there ya go, just connect a battery charger to the generator and use that to charge the gel batteries. If you're going to charge the gels off the truck's alternator, then you'll have to figure out a way to separate out the gels via an isolated, programmable voltage regulator that is programmed speficially for gel batteries, or figure out some other way to get one alternator to output two different and correct voltages at same time.
Or, just hook 'em up with an isolator or combiner and hope for the best. Just don't be shocked when they work one day, but not the next. If you're lucky and they aren't overcharged even that one time, then they should last a long time.
