I treat our RV's two 12V Interstate coach batteries about the same as you describe - except for the watering. Our Interstate batteries have no liquid acid in them because they're AGM batteries.
Ours are about 8 1/2 years old and will still get us through a couple of days of dry camping before needing charging. However, we'll be replacing them soon with a new set of AGM batteries. We keep them charged when the RV is at home, and mostly when camping, using the stock converter in the RV. Between campsites the engine alternator pumps high amps into them and can bring them back up to full in about 4-5 hours of driving. Our old Interstates actually seem to perform slightly better after we've exercised them a bit during the the first few days of a camping trip.
These AGM batteries are deep cycle 12V batteries - not the common marine/starting type of battery. Your Interstate battery may be the marine/starting liquid acid type? If it is, you might want to consider replacing it with a true deep cycle type of battery.
