There are many reasons.. Some things you do not want running on the inverter cause they draw too much power.. And the idiots who wired my RV put the bedside outlets and the Refrigerator (Absorption Cooling unit) on the same circuit so ... (more later)>
Generally a small (like 500 watt) inverter only powers the entainerment stuff,and many times not all of that. (only main room). Up it to around 1500 and we add the Microwave.. another 500 (2000 watts) the GFCI (So you can use your electric shaver) but I found that my engine does not charge the batteries fast enough to power the 350 watt fridge.
I ended up putting in a manual transfer switch.. This is a proper 30 amp Transfer switch (The store did not have a 15 amp) double pole, double throw, center off.. To transfer the bedroom to INVERTER I first turn it to OFF (You can not go ON to ON) then turn off the "AUTO" feature on the fridge (This locks it on GAS) then I "hit" the transfer switch again moving it to inverter..
Switching back I double tap it back to mains and tap the AUTO button on the fridge so it switches.
