Not sure about why the particle board is there, doesn't really matter, the factory uses a lot of junk filler pieces as it is not structural.
The most important thing is where you had water infiltration. Did you find the leak on the exterior?
I am sure you already know....That size hole on the interior, in that location is going to be a nightmare to fix.
Start by stopping the water. Look at marker lights, roof seems ect.
2011 Coachman Freedom Express 295RLDS- Sold
2015 F-250 FX4 SUPER DUTY 6.2L Gas 4WD SuperCab XL 3.73 rear / 158" WB / 8' bed. Factory anti-sway and brake controller