There is little advantage to 24 volts, especially since the existing controller can easily accommodate 400 watts.
If at a later date, you wish to move to 24 volts and add more panels that option is open.
I would go with 2 SiO2 100 amp-hour batteries wired in parallel, for these reasons.
1. no special chargers are required
2. they can be taken to 0 volts without damage, after 620 times 80% of oem capacity is still there.
3. they can be used in extreme cold.
4. they are 1/2 the price of Li.
For 24 volts if you place two 12 volt batteries in series the number of amp-hours remains the same--but the number of watt-hours of capacity doubles.
For 12 volts if you place two 12 volt batteries in parallel the number of amp-hours doubles--and the number of watt hours of capacity doubles.
If you move to 24 volts, the sensible solution would be to get a DC to dc converter that is capable of running all the 12 volt loads in the RV.
This series of articles may help you understand solar better:
https://freecampsites.net/adding-solar/In your case my personal choice would be to stay with 12 volts.
