The advice your dealer is correct. I'd check the water level weekly or perhaps bi-weekly. Carefully take a medium sized screw driver and pry off the 2 rectangular "caps" on top of the battery. A flashlight will help you as you look down the 6 holes on the top of the battery leading to interior of the battery. You'll see the "water", actually battery acid water, down inside the battery. This acid is corrosive and will burn your skin so be very careful. If you should get any of the acid/water on you, rinse it off immediately. DO NOT touch your eyes or face if you suspect any acid/water on your hands or fingers.
As you look down each "hole", if the acid/water is up to the bottom edge of the hole the level is correct. If the acid/water is below the bottom of the "hole", add only enough DI water to bring it up to the bottom of the hole. Each "hole" is a separate compartment or cell of the battery so each one must be filled individually. Clean any spilled acid/water from the top of the battery and replace the rectangular "cap" and trap it down with the handle of your screwdriver and you are finished with that battery.
Do not overfill the battery cells as they will, due to the chemically action of the battery, overflow which is messy, will corrode metal, and burn your skin if you touch it.
As just want to expand on what you see as you look down each battery hole. The hole is actually a cylinder that goes perhaps 3/4-1" down into the battery. So when I say to the bottom of the hole, I taking about the acid/water touching the bottom or lower lip of the cylinder.
Hopes this helps! It's pretty straight forward once you start the process!
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's