I won't answer #1 because I don't know anything about standards for small RV batteries. But I'll give an idea about it in #2.
#2 You can use a diode instead of an isolation switch. It prevents flow backwards into your starting motor.
You definitely need a charge controller. It will automatically charge your battery and disconnect when the battery is fully charged. I think a good one could also limit how fast the batteries charge so as to not overload your alternator and also that upper limit would tell you exactly what size wire you need. So I won't comment on whether #10 wire is adequate, except to say you must insert a fuse in the #10 wire line, so its current rating is not exceeded; ie. the fuse will blow if it is exceeded.
The amount of explosive hydrogen released increases as the battery becomes fully charged. So you can see if you have no charge controller to cut off the charging, then huge amounts of hydrogen are released and the water level drops in the batteries. Even with a charge controller, of course explosive hydrogen is released, but stops at full charge.
You might not need the diode or isolation switch, if the charge controller prevents backward flow.
#3 I think having two inverters offers no advantage except differences in efficiencies, because current drain depends mostly on the item being used. I would just buy a single good quality efficient large inverter, rather than two cheap poor quality ones.
