That hybrid checklist was going to be my first suggestion. Glad you found that because it will be a big help for you.
I think its all about water intrusion. Roof, walls, floor, etc - are they spongy? Discoloration or other obvious issues of water? Check the seams on everything on the roof.
The bunks usually leak because of the seals so check those. Its 5 years old so pay attention to that. Are they still able to maintain a good seal or do they need to be replaced?
Also, I can't recall the year, but it was around then that FR changed up some things with their bunk ends. There was a retrofit kit that owners of older units installed. I would do a little more research on older threads regarding that fix to see if it applies to you or not. I'll try and find more but am traveling right now.
With everything else, run it all through its paces on both AC and DC. Use both the FW tank and pump and city water to check for leaks.
I hope this helps you. When we have bought used in the past, two or three people and one dealer were hiding problems that were pretty obvious once we dug into the camper. 90% of the problems can be caught if you go into it with a jaundiced eye. However, we actually found that most of the issues new owners usually deal with had been corrected so these were great buying experiences.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD