Ok, NO power TO the fuse panel,,, That's actually a good thing (It strongly narrows down the choices)
First question: No power ANY TIME or no power when not plugged into mains?
I know the 2005 Damon better than yours so here is how my rig works
I have a switch, it is under the hood of the motor home, right about in front of me when I'm driving but on the engine side of the firewall. Wires run from it to the battery, and from it to the other battery, and from it to the power panel and one assumes the converter and generator.
Chassis battery runs headlights and marker lights and the like, (Vehicle lights) ciggarate lighter jack in dash, Engine starter, and other "Dash operated" things
House batteries run everything else, via that switch, There may be a master fuse.
If you have the test lamp, hook up some test leads and do this, One lead each, Positive and negative battery, GOt light, GOOD, no light battery issues.
IN all cases, when the light don't light you have a clue
Next step, move negative lead to chassis ground (any hunk of metal)
Next step is the battery master switch, As I said, in my rig it's driver's side, top right, there are fuses there too
Move to the power panel, Same test (use chassis as ground, you may have to reach some to find a ground here)
Let us know where the light don't light
NOTE: there are many types of battery switches, Mine has 3 very heavy leads (Chassis, House, Load) all 3 should light the light when the other test lead is connected to any chassis ground