The real secret is knowing what is ACTUALLY going on inside the battery. When your hear "bubbling" what is the voltage and what is the amperage? Is the hydrometer reading EVEN across all six cells or is it uneven?
It is very common to hear slight bubbling when the batteries are being charged at an amperage reading above 50 and a voltage up to 14.2 Time is money and your alternator voltage regulator prevents the pair of batteries from being charged "too fast". If your rig has a cigarette lighter socket buy a cheap DIGITAL plug in volt meter and read the voltage after charging. Wait 2 hours. The reading should be 12.7 volts, with no other loads on in the rig. If it hasn't reached 12.7 fire up the car again for another hour. A gallon of gas is cheaper than 2 batteries! If you try to read the voltmeter while lights are on, inverter or other load the voltmeter reading will be falsely low.
This is an acceptable regimen for a pair of golf car batteries that get used heavily on a 2-day weekend. On a 3-day weekend, charge them Saturday and Sunday. If your tow vehicle has a good model alternator it's going to provide a mid-rat snack your batteries will love.
Charge the batteries back up fully and completely once you get back home. Don't let the rig sit several days without recharging. Once again, time is money. BFL13, and Salvo seem to know of a good converter charger in case you don't have one. But a billion dollar charger is junk if it cannot get the cells back to fully charged without jumping through hoops. Good Hydrometers do not lie. A good one is a FRANCIS FREAS. Google it.
Sulfated batteries WILL BUBBLE even though they are not fully charged. The heavier the sulfation the more intense may be the bubbling. But a hydrometer tattles and tells you the truth.
YOU DO NOT NEED to walk around with the hydrometer stuck in your back pocket. I use mine only when I suspect my instruments are lying to me. The greatest lead acid battery POLYGRAPH MACHINE ever made!
