For anyone considering Hydrocaps, they sound like a good solution to outgassing, if you have relatively new batteries.
However, if you are going to buy new batteries soon, I highly recommend that you consider Lifeline AGM batteries. AGM batteries are sealed (no watering required), do not outgas at all, and cause not corrosion at all. The only drawback is the additional cost. However, for the cost of the Hydrocaps, apparently $60 per battery, that would cover about half the additional cost of a Lifeline AGM battery. In addition to no outgassing, the Lifeline AGM batteries have other benefits:
1. Faster recharge
2. Lowest discharge rates when unattended
3. Shock and vibration resistant
4. No battery watering required
I have the Lifeline AGM batteries in my coach, and I swear by them. Check 'em out at this link: Lifeline AGM batteries