Charge spare battery

My wife and I both use CPAP machines. When we are at electric sites, no problem. When we do an overnight without hookups, it becomes a problem. The CPAP batteries run over $200 each, so over $400 for us both. I already have an inverter big enough to run both machines. I bought an AGM battery big enough to run both machines through the inverter for a night, but am unclear as to the best way to recharge it when I get a plug in site. Can I jump it to my camper battery and let the converter charge it, too (Progressive Dynamics converter), or do I need a stand alone charger? Thanks for your help.