First the easy part.
You should not have to upgrade cables or converter. You will need to get additional cables to connect the multiple batteries.
Now the more challenging part, I am going to assume your coach has one 12 volt battery. If that is the case you can get an additional 12 volt and connect the two batteries in parallel.
If however your coach has 2 6 volt batteries you can add additional batteries and the batteries with then have to be connected in a series-parallel configuration so that the voltage entering the coach is only 12 volts.
Hopefully this helps.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054