interest question

Only if your pattern of use requires the additional capacity. If you need two, maybe also you need some recharging capability, like solar or a generator. Or maybe just a larger battery (larger single batteries have roughly twice the capacity of the smalkest provided with RVs).

If you are finding your battery run down when you need to use it, either it is not large enough (you need something bigger or more of the same) or it is worn out and needs to be replaced.

I have two smaller batteries in parallel, seldom use the caoacity of one. The positive side is that when one goes bad (open circuit or shorted cell) I can rewire tonuse the other. Downside is that when one is weaker than the other, it cannibalizes from the good one, and confuses the charging circuits so neither really gets charged to capacity. But if I figure this out, I can wire the bad one out of the circuit.

Only if needed. Why do you think you might need two batteries?
Maybe you need four. Maybe a 100 watt solar panel would be more useful.

Two ways to read your question - 1) Is there advantage to parallel as opposed to series batteries? Sandia Man answered that.

-or-

2) Is there an advantage to adding a second battery to your Rockwood? In that case it depends on your type of camping - if you are always on shore power, then no, one battery will do. The only reason you need a battery then is to power the emergency braking system and for lights for the short period of time you are not connected to shore power. On the other hand, if you boondock, then yes, adding a second battery will help.

Sure, more amps. Batteries in parallel have their amps added together while volts remain constant. Just the opposite happens with batteries in series where the voltages are combined and amps remain constant. Dual batteries are always better than a single battery no matter which wiring method is used to achieve 12 volts.