In my Sunseeker 2300, the transfer switch is under the bed. Access by removing 4-6 screws and lifting off a half sheet or so of plywood. I've not had any trouble with mine as of yet, but since I'm a retired industrial electrician, exploring the electrical systems was high on my priority list! If your 120 volt/12 volt power box isn't in the bed base, look for the power change over relay somewhere nearby. It is a metal box around an 8" cube, and is marked as to what it is. Since you have checked all the breakers and fuses (all of the 120 volt stuff is on breakers)but still have no power on shore or generator, double check your main breaker (30 amp) You have to push the breaker handle firmly to the off position before you can reset them, just like home breakers. This should at least get you in the ball park!
A really handy tool to have is a non-contact circuit tester, available from Home Depot or Lowes, or any electrical supply house. It will light up and beep when the end gets near a wire or connection that is live, even through the insulation. This saves a lot of time, as well as shocks!