You said the fuses "look good" but I have had bad ones look good. Connect the black lead to a good ground and measure to both sides of the fuses, should have power on both. It could also be an open ground circuit. Verify continuity from the detector ground back to the main ground.
I've also found where one manufacturer used the white wire as the ground while another used the black wire as the ground. Verify which way your camper is wired.
I've bought two campers that the detector had no power. Both were because the previous owner had disconnected them to keep them from alarming while trying to sell the camper. I connected the wiring and after a while the alarm would sound momentarily.
One had the oven pilot turned on without being lit and the other had the flexible gas line leaking at the regulator.
I also had a 12 volt circuit that didn't work where someone had tried added a wire for a fan, but the original wire had slid out from under the screw while tightening the screw.
Good luck