The Samsung RF18 at 24.5" width with doors removed should fit through your Bounder 35E door opening that's slightly larger than 24.5". However, precise measurement and careful planning are essential for successful installation.
Onan code 36 indicates a low fuel pressure issue in LP generators. This typically means a problem with the LP regulator, fuel solenoid, or gas line obstruction preventing proper fuel flow to the generator.
Replace failing N3150 with NovaKool R4500 compressor for better heat performance or Norcold N3141 for absorption drop-in. NovaKool better for boondocking with solar, absorption for propane versatility.
Frigidaire FFET1222 dimensions are generally accurate. Allow 1-2 inch clearance on all sides for ventilation. Measure your opening carefully including door swing and handle depth.
Replace Atwood GC6AA-10E with Dometic G6A-8E - its the same heater under new branding. Dometic acquired Atwood so mounting patterns and connections should match directly.
This sounds like a main electrical supply issue, likely from your shore power connection or inverter system. The E7 codes and simultaneous failures point to insufficient power reaching your high-draw appliances.