First place I would look: your power cord goes into a junction box in the storage bay where the cord is kept. There, each of the three wires is tied to the wires that feed the AC panel is connected to the power cord with a wire nut. One or more of these connections may be partially loose after 10 or more years of handling the cord.
Second place: the plug on the end of the power cord can get damage enough that it intermittently carries now power, depending on the quality of the connections in the socket where you plug it in. Symptoms include a hot plug, scarring and blackening of the blades. At ten years, I'm on my third plug, and now carry a spare.
Third place: inside the AC panel the neutral connections come into a bus bar where each circuit his held down by a screw. These loosen. If you have AC circuits working yet the 12V power is intermittent, check these connections, the one for the 12V converter may be loose. A loose connection on the hot side is less likely, as the breakers clamp in. I'm assuming you know how to check whether a breaker has tripped.
Fourth place: loose connections on the output side of the converter, either hot or ground. Or possibly a connection at an in-line fuse on the feed to the batteries.
After checking connections, then look into intermittent failures of components like the battery cutoff relay, and the converter itself. On my 2004 Itasca Spirit, that cutoff relay is energized from the house batteries, so if they go down too low with the relay latched in "off" position, it cannot be flipped to "on" without charging the batteries or supplying an alternate 12V source.
If you would say whether or not you have power on the 120V side, we might be able to skip checking the first two trouble spots.
Tom Test
Itasca Spirit 29B
