New subfloor for new hardwood floors

[joe77]

Hello,
So i'm looking for some opinions on sub floors. I have a 77 argosy (all aluminum floors.) Some areas have sagged so i'm needing a level floor to install hardwood floors. I'm leaning towards 5/8" CDX, mostly for a cost reason.
I'm also wanting to secure the sub floor to the ribs running under the aluminum floor. They are 2" wide and 48" apart. The issue i have is securing the CDX to the ribs. I only have 2" of space to secure two pieces of cdx. Thats 1" of space for each sheet. I'm afraid i will chip and break the cdx trying to dill in 1/2" from its edge. Any suggestions on how to secure the subfloor?

If i use 12'x5' sheets as apposed to the typical 4'x8' I'll avoid having to secure two pieces to one rib, but I'm not sure i can get that 12' piece through the door!!!

Any suggestions on best method and materials for subfloors???

Thanks
Joe