I have 5 panels 265W 31v I want double the system to 10 panels for an off grid cabin about 2500-3000 watts
I read to make the system 48V is preferred to 12v.
I need to buy new batteries so it’s a good opportunity to buy the 48’s
If I wire the 31v panels in series that will give me enough to charge I believe.
So I was going to join the two arrays into one controller.
5 31V panels in series should be OK for a 250V MPPT.
As you have a 48V battery the solar panel voltage needs to be higher than this so you can not use tjen in parallel. Different MPPTs have different requirements of minimum PV voltage. Putting them in series may or may not be OK for a 250V MPPT. You could use 6 in a 3S2P arrangement.
Again, try the MPPT calculator, it will suggest suitable controllers.
You can combine different panels on one controller of they have separate trackers on them like the mppt RS series.
Otherwise separate charge controllers are more optimal when the string have different mpp and imp values.
Its not that it wont work, its just that performance will be hurt.