Here is what to expect for recharge times from a smart charger on your two batteries starting at 50% SOC, using various charging rates.
Do not imagine the 13.8v 7345 will do anything close to this! :(
You can see how tapering starts sooner at lower SOC the higher the charging rate using the same voltage 14.5ish. but that early tapering still has the amps higher than the constant lower amount, so it is winning until they become the same. After that the time is all the same. Time savings is all up front and to get that you have to start at a low enough SOC or there is no point in having a higher amp charger.
Your problem is if you only want to replace 50AH (not amps, it is AH) and your have a 50amp charger, then you must start at 50% but in an hour you have only put back 23% of that 220 so now you are up to 73% when you stop the gen. Unfortunately, you can't just keep doing that!
Leaving the batteries undercharged lets them sulphate, so they get you coming and going. Good news is that for a few days camping , like a long weekend, you can do that.
On the generator vs shore power, it usually doesn't matter. However, some gens like the 4000 Onan (and maybe only some of them) have a "dirty" waveform so it is not "clean power" like shore power and then some (but not all) converters will not run as well (not to fuill rated amps) but they will still run. The usual inverter-generators like the Honda 2000 etc are clean power.
Then of course you have to have enough generator power to run the charger where a high amp charger wants lots of input power, so there are limits there.
To achieve a desired generator time to do x AH back in, you need the right charging rate for that size battery bank at the right voltage, to start at the right SOC, and have a generator that can run that charger that long. Now all you need is to be in a campground that allows you to run that generator that long. Some don't allow that.
