While I have only one coach battery and have no problems to date, I would suggest that anything you read about parallel connected batteries will warn you of eventual problems. If the two batteries do not have identical voltages, the stronger battery will try to force current to run through the weaker one. Since the narratives in this chain put forth all sorts of reasons why one battery is often not as strong, you will tend to overcharge the stronger battery trying to maintain the weaker one and eventually it will fail. Many sources suggest wiring two 6 volt batteries in series to replace dual 12 volt batteries as a more reliable, longer lived solution. The best technical answer it to install a solid state isolator that will selectively charge the lower voltage battery and not keep charging a fully charged battery. These are not very expensive, maybe $30-$40 or so. The real problem is that battery charging is more complicated than it looks. The fully charged voltage of a "12 volt" lead acid battery is more like 13.8 volts. How quickly the battery loses voltage depends on how much current is drawn, the condition of the battery, the electolyte level, and the ambient temperature. Also, chargers usually have a maintenance voltage setting which stops the charging as long as it is maintained. If this voltage is too high in your charger, it will tend to overcharge the battery, which will shorten its life expectancy. Some sources suggest that a stored unit with good quality batteries needs to be charged only once a month or so, and that leaving the charger hooked up at all times is not the best practice. If you don't mind the hassle, taking batteries out when they're not being used and storing them in a climate controlled area will extend their useful life, as Ron suggests.