The external cover has hinges just like the basement doors. Remove 5 or 6 screws, and it swings upward to access the the unit. There are large bolts (as I recall 4) that hold the unit to the chassis frame. There will usually be shims on the bolts(about 1 inch square) that come out when you pull the unit out. Make sure you know how many are at each bolt so that you can replace them when re-installing.
Make sure that the shore power is off before removing the unit. The wiring that comes in towards the rear can be left connected if you carefully swing the unit out at the front, leaving the rear supported by the chassis frame. The front of the unit can be supported on a base that will support 200 pounds - Strong crates, a step stool, etc.
You can then remove the unit top cover, power it up, and inspect for problems.
I have a PDF copy of the service manual. If you PM me, I can send it to you via e-mail.
2008 Itasca Meridian 37H & 2015 Flagstaff T12RBST
2011 & 2012 Len & Pat's "One lap of America"
27K miles & 41 states in 13 months - Woo Woo
Yellowstone Lake 6-1-2012