You should look for many of the things as you would for with any vehicle 12 years old, dry hoses, belts, etc . I have the residential refer in our 2005. The original inverter was a modified sine wave, lead to problems with the refer control board, WRV had a modification that fixed it (see thread on this forum).
You also should ask what has been replaced, inverter, AC, refer, etc. You can also look at the coach VIN, in the stickies at the beginning of this forum is a register of all VINs and the owners that have identified themselves. This forum is also a great source of help.
Dave, Bobbi and Fenway
2005 38' FDTS Alpine Limited, 2016 Jeep Grand Cherokee Limited
Cape Cod, MA