you should be fine to leave the charge on. It will not turn on unless it is shore power plugged in anyways.
yes, not all chargers are equal. Unfortunately the better ones do cost more money.
also, "wet" lead acid batteries require maintenance and more frequent inspection. Over charging will cause the acid/water to boil and this gas will evaporate, you need to refill with distilled water. if you don't and let the fluid level get too low, the cell(s) will be destroyed and can not be fixed.
1. take a look at your charger and determine if it is will do the task and maintain the batteries properly. chargers need to be powerful enough to "wack" the sulfation off the plates, smart enough to know when to lower the amps to not "boil" the cells and float/trickle charge to maintain the battery from slow self discharge. Generally you need about 8-10 amps charging capacity per bank. If you have 2 batteries: 15-20 amp charger, 3 batts: 20-30 amp charger, etc.
2. make sure the batteries are marine grade (much heavier plates and construction than car batteries to handle the vibration and abuse)
3. consider switching the batteries to AGMs (absorbed glass mat) as they self discharge much slower, no maintenance (water to check and fill), generally last longer in marine envirnoment since there is no fluid to "slosh" back and forth in the cells and the plates are packed against each other in fibreglass, generally last longer than wet cells because no maintenance is required--albeit generally more expensive than wet cells, but hey if they last longer that should even out the score.
my grady came with a Charles charger and it still working very well. had a few questions and their service was very good too.
