It all depends on what you're trying to power up and how much your batteries can provide.
To put this simply. Battery power is measured in Amp Hours. That is, how many Amps it takes to drain a battery in 1 hour. (hey, I said simple! don't jump on me for that).
Lets take an example. A car stereo typically draws around 3 amps. in theory, a 90 Ah battery could hold that up for about 30 hours if that is the only thing connected to it.
Something to keep in mind is that when running through an inverter, a device that converts 12 volt DC into 120 Volt AC, the amperage draw from the 12 volt side is 10x of the 120 volt side.
So a 1 amp device on 120 volt draws 10 amps on 12 volt, not accounting for losses in the conversion.
So the bottom line is that you need to learn what is running on the batteries and how much power they draw. Then compare that to the capacity of your batteries.
Typically there are 2 or more batteries in the entire motorhome. 1 for starting the engine and powers normal things you would in any vehicle (headlights, etc). and then 1 or more for powering everything in the coach. The coach batteries can be run completely dead and you should still be able to start the engine.
Now, I've seen generators connected to the coach batteries, some connected to a dedicated battery, and then others that are connected to the engine battery. So whether or not you could start your genset when the coach batteries are dead depends on how it is wired.
An isolator is in place to allow your engine's alternator to charge all batteries while driving, so even if you can't start your GenSet, you should be able to just start the engine and let it idle for 15-30 minutes and then the GenSet should start.
So the coach batteries will recharge in these ways:
1. While plugged into "shore" line.
2. While running the GenSet
3. While running the main engine
If you have solar, those will charge the coach batteries as well. Solar panels can either be manually switched or automatically switch, again, that depends on your configuration.
