The Flow-Rites seem expensive, but they saved me money.
When we downsized to the PleasureWay I found that it was intended to have AGMs. The compartment was small and awkward. The month ours was built, there was a shortage of AGMs in Canada. We got wet cells. To service the batteries, they had to be removed.
Rather than buy AGMs, I "saved" money by getting the Flow-Rite Quik-Fill system. It worked extremely well. Besides making the batteries easy to service, the venting is somehow different. In three years of use, the tops of the batteries never showed any acid or other wetness. Consequently there was no corrosion. None! And, that can be a money saver too!
https://www.flow-rite.com/battery-care/brands/pro-fill-rv/ or on Amazon search for the "Flow-Rite RV2000" for 6 volt batteries or the "Flow-Rite MP2000 Qwik-Fill" for 12 volt.
FWIW, Marty gave me a tip early in the Monterey ownership. I was loosing the corrosion battle. Baking soda and battery cleaner were not keeping up. With the sliding tray there were too many moving parts starting to corrode. Marty suggested hosing the batteries and tray down with water whenever parked on gravel. I replied that electricity does not like water. Turns out that wet cell batteries are always installed in open space where water can get to them anyway. Marty was right! They don't mind a fresh water bath! I put an adapter on the outdoor shower so, using the fresh water hose, I could spray warm water. That seemed to work better than cold. When we sold it after 12 years, the battery tray and the wires were all "good as new"! Remember the gravel part. If you do it over concrete, it will be permanently stained.