What is the best way to charge my batteries?
The people that make batteries know that lead and sulfuric acid will store electrical current. That is what batteries do. They STORE electrical current, they don't create it.
The people that make batteries also discovered the best way for the battery to store that energy the longest is to put the electrical current into the battery in 3 stages. Bulk, Absorption and Float. They apply the electricity to the battery at 3 different voltages for 3 different amounts of time. Each manufacturer has slightly different spec's for how they want it charged.
When we use the battery we need to put that energy back. The very best way is to duplicate what the manufacturer did. In 3 stages.
For a starting battery a typical alternator charges at 14.4 volts and in one stage. The starter needs the battery to be at least 70% charged to work. The alternator then puts the charge back in the battery. This has worked for many many years, even though for most of those years the battery is only getting one stage of charging, and typically at lower voltage than is best for the battery.
Some newer vehicles have 3 stage regulators in addition to the alternator that do try to do the job of the 3 stage charging. But overall an alternator is really only any good at recharging the battery in the bulk stage, and even at that, it is lower voltage than the manufacturer of the battery used to store the energy in the first place. This is why sulfation and crud build up happens on the battery. The lower voltage can't zap the crud off.
A 4th stage of charging is also possible. This is the highest voltage of the all the stages of charging and is known as an equalization charge. One of the things this type of charge does well is to knock the build up of crap off the lead plates.
Going back to your vehicle battery, as long as your alternator is putting out what it is supposed to, despite the battery not always getting the 3 stages of charging that is best for it, a half decent starting battery will last around 5 years in this climate.
A deep cycle battery is different. It really needs the 3 stages of charging to stay healthy. A cheap battery charger only does one stage. A cheap converter / charger in your RV only does one stage. A cheap solar charge controller only does one stage. And even at that, the voltage is likely to be too low to do a good job. So your deep cycle batteries will fail early if you reply on any of these ways to charge your batteries back up.
The size of the battery matters too. If you have 4 giant batteries, they will never recharge properly unless there is high enough voltage and high enough amperage to make them do so. Depending on the batteries this could mean needing as high as 40 amps of charge. Your car alternator might give you 2 amps and that's AFTER it does it's job for the starting battery. So no, your alternator is NOT the way to charge your camping batteries.
YES it is better than nothing. Anything is. Don't baby your batteries. Use them. Charge them right away. Doesn't matter if they went down 10% or 70%. Charge them right away. Make them work. Batteries hate sitting. They discharge by themselves hooked up or not. We have all heard it's best to trickle charge a battery. This is true in a few cases. But over time only trickle charges are nowhere near enough amps for the battery to stay healthy.
Use the trickle chargers and maintainers to store your RV batteries. AFTER you are sure they have the proper voltage and amperage to get those batteries bubbling and happy.