Specific advice depends on the specific refer. They do not all act exactly alike.
Mine will work on all power sources, but does not always transfer as easily as it does in the brochures. One thing is that when going to gas, if the gas had been off for a while, you may need to allow the air out of the lines. (I know I do)
Many of these refrigerators do not automatically switch, some do. If yours does, it will likely only use one source of power even if you provide it with more than one.
There should be a priority list or order it uses power in the paperwork. An example is that if mine is running on gas, and I plug in the shore power, it will automatically switch to 120 volts. Then when I un-plug, it will go back to gas, as long as the gas lines are charged. (If there is air in the lines, it will eventually go to gas, but the warning light comes on for a while)
The fact that you pop circuit breakers, and are asking how long it takes to cool however may be more related than you think. If all is okay, and just taking too long is very different than popping circuit breakers. I do not know your details, but in my experience, things that run as expected do not pop circuit breakers.
So, find the owner's manual for that refrig, and look for details there, find the details on that 15 A circuit breaker to be sure exactly what it powers, and find whatever the short is, and publish the model and details of the refer here for more exact assistance. Even a picture of the back of the refer (inside the access door from outside) is a good idea to get more specific help.
Karl I. Sagal KarlSagal@Gmail.com
Well done is better than well said. (Ben Franklin)
1988 Fleetwood Southwind, 34'