Whatever is the lowest point is the limit.
You can add stringers to allow attachment and support, use a 2x4 as a guide to keep them at same level.
Between stringers glue in foam board.
Add conduits for wires to lights.
Visit hardware department, brass screws with brass trim washers look nice, do the job well, not expensive and allow removal later.
Trick is spacing and alignment, mount panels bare to allow chauk line to make grids for screws.
Once all are done mark panels, then covering is added, rolling over edges.
If seams are good no trim needed.
Or you could try single piece seamless, or add brass inter-panel "H" trim that is flush and requires no hardware
Can use paneling, luan or padded panel, many oprions there.
Ours is padded vinyl, looks real rich, southwind has what looks like short nap carpet, others have wallpaper looking stuff.
Since you bump your head it needs to be non staining and washable
Maybe visit a well stocked wall covering store that have folks who know what they are doing...not home depot...
They may be able to give you some good ideas.
Tony & Lori
1989 Country Coach Savannah SE