Typically the brakes are sized according to axle weight.
The drum size gives you an idea as to the axle weight.
Being you have a 5th wheel, you could have 10" drums for 3500 lb axles.
You could have 5,000 lb axles which use larger 12" drums.
But that is assuming that you have Dexter or Dexter compatible axles..
Very good possibility you may have Lippert axles and not all of theirs are 100% compatible with Dexter parts..
Lipperts 4,400 lb axles are one of the oddballs which you may run into that only Lipperts parts will fit.. They look like 3,500 lb Dexter axles with 10" drum but the backing plate has a different bolt placement.
If you have the original drums perhaps see if you can find any casting part numbers and or manufacturers name which may help narrow this down.
Axle manufacturers were supposed to include a tag on one side of the axle which states brand and axle weight specifications but that tag often gets lost with time.