[Motor31]

Quick and dirty answer is a multi meter at the battery. If the battery(ies) charge level shows 13 volts or more it is almost certainly being charged. Then you can push in the battery isolator, wait a couple minutes and check again. If the charge level drops then the inverter was definitely adding juice to the battery(ies).

My old style converter uses a 13.8 volt level charge. My batteries are now on their 7th year of service although I think I'll be changing them this before fall. One is showing more than just 1/10 volt difference in holding level. I use two 12 volt deep cycle batteries in my house bank. I check the batteries every 3 months and add water as needed. So far I have not had to do anything more than that. Unlike my car and tow vehicle batteries, the trailer batteries have not caused corrosion at the leads.