Old thread but Ted revived it not me!!
To the best of my knowledge, most RV battery chargers are single bank chargers (such as https://www.progressivedyn.com/rv/po...vertercharger/
). However; there are some that do have a second circuit for the chassis battery. I've not seen one intended for the RV market that has more than two circuits - but would not be surprised that they exist.
There is some information here that is worth talking about. All of the following assumes we are talking about flooded lead acid (FLA) batteries.
1. Voltage is a terrible way to measure state of charge. It only sorta works after the battery has been resting for many hours (like 24). That never happens in typical use. Using a battery hydrometer to measure the specific gravity of the electrolyte is the only accurate means to measure state of charge of a battery. Since that is horribly inconvenient, the second best way is a quality battery monitor with a shunt that measures all the electrons flowing into/out of the battery. The better ones allow some configuration to fine tune. If you are operating your system using only a voltmeter, you fall into the category of people that are abusing your batteries. Sorry...
2. For typical use, more than one battery bank is just wasted effort. I agree, the concept sounds nice - backup and all that. However; if you properly care for your battery bank, it will take care of you. They do not fail without warning. If you ignore them, don't water them, discharge too deeply, overcharge, etc..., yes, it may fail when you least expect it. Don't do that. You wouldn't do that to your engine - why do it to your batteries? I would argue that properly maintaining your battery bank is time/money better spent than adding redundancy so you can ignore them. That said, if you have the knowledge and budget to do that sort of thing and it makes you feel better, by all means - do it. I just hate to see people with little electrical knowledge thinking this is the way to go, fumbling with how to do it, and then using it as an excuse to omit proper maintenance.
Assuming you have a battery bank with several batteries (more than two six volts), you already have a backup. If one battery does go bad unexpectedly (it shouldn't - you need to do more maintenance) then you pull it out and reconfigure the bank. Four 6V batteries would become a bank of two 6V's, two 12V batteries would become a single 12V, etc.
3. The notion of 'charging one battery while using another' is not useful. There is no such thing as charging a battery while consuming power from it. It is either being charged or discharged. For our type of use, a battery may be under charge while house systems are in use. When that happens (common), the charging source is providing the power to both. For example, a charger from shore power may be sending 20 amps at 13.6 volts down the line. Ten of those amps (for example) may be used by the house and ten of them may get into the battery (ignoring losses and all that).