There are four common ways of charging your RV batteries.
The first is by driving. In a motorhome, your house batteries will always charge by driving. If your batteries are down to the 50% level, it will take about three hours worth of driving to bring them back up to full charge. In travel trailers and fifth wheel trailers, your house batteries will charge if your trailer wiring connector is wired to provide that charge. But the charge will often be slow and not complete due to the smaller wire sizes provided by the connector.
Second, you can simply plug in your RV at a campground. Every RV has a built in converter/charger unit to convert 120 volt household electricity into 12 volt power. The converter side of the box provides 12 volt power to the 12 volt circuit in the RV, and the charger side of the unit provides power to recharge the house batteries. Usually, your batteries will be fully charged by morning.
Third, a generator. Most motorhomes have a 120 volt generator on board. Using the generator is exactly the same as plugging in at a campground. The power from the generator goes to the converter/charger box and the batteries charge exactly the same way as if you were plugged in at a campground. Except that you get to listen to the annoying drone of the generator. Many people with travel trailers or fifth wheel trailer also carry around a portable generator.
Fourth, solar panels. I could write an entire blog post just on solar panels, but again we're trying to stick to the basics here.
Let me tell you a story. Our first time in Mexico with our motorhome, we had cheap Walmart marine batteries and we did not yet have any solar panels. I had been relying strictly on driving to charge the batteries. I was also occasionally running our built in generator, but we can't stand the noise from a generator. We were parked up in one place for about four days in a row and the batteries were very low. I drove into town, about 15 minutes each way expecting that to charge up the batteries. Well it did. I think it brought them from 30% charged up to maybe 40% charged. It was then, that we learned how important it was to our style of RV'ing to invest in solar panels!
Of course the drawback with solar panels is that they will only charge your batteries when the sun is shining.
Speaking of weather, this is very important. The usable power in your batteries goes down the colder the temperature is. They say that 80F is the optimal temperature for your RV batteries. For every 15F drop in temperature, the usable capacity of your batteries drops by 10%. So when it goes down to 35F overnight, you have to remember not to draw your batteries to low by using your RV furnace very much. The fan blower motor in a typical RV furnace sucks a lot of 12 volt power!
Battery maintenance. Two very important things here. Keep the battery connections clean, and keep the fluid in the batteries at the proper level.
Most cheap marine batteries are sealed. These batteries will typically expire just after the warranty does! Nothing that you can do to change that short of making sure you don't use them very much. Of course you still have to keep the terminals clean. Still, these batteries are fine for those of you who move from campground to campground and are plugged in all the time at those campgrounds.
Deep cycle wet cell batteries have caps on the top. You have to pop the cap off, and check the fluid level in each cell. When they are new, most deep cycle wet cell batteries don't lose very much fluid, but as years and use go on, they require more frequent inspection. You need to "water" your batteries with distilled water, never letting the fluid level go below the plates.
AGM batteries are completely maintenance free. It's one of the benefits of the price that you paid for these high end batteries!
Last year, I bought one of these little battery system monitors that simply plugs in to your cigarette lighter. On our motorhome, the cigarette lighter is supplied by the house batteries, so it gives me a live reading of the battery at any given time. And, it's on sale right now. For the money, you can't go wrong!
Battery and Charging System Monitor