Plug your fridge into an extension cord to see whether it will run on 120v (it uses an electric element to heat the gas as opposed to a gas flame, but the reefer will (or will not) work no matter which you're on). You'll need to leave it plugged in at least 24 hours, since gas-absorption units are very slow to cool.
No point in going further if the reefer doesn't cool.
If there is simply no power to the outlet where your fridge plugs in, check the breakers in your power center, and make sure there is not a secondary GFIC outlet in the hall, outside, or somewhere else that is popped. At the very worst, you can run a new 120v circuit for your fridge, or even a permanent extension cord.
**edit- I see you've said the entire passenger side is out. This is most likely a) a tripped or bad GFIC outlet *most likely* b)possibly bad or tripped breaker (easiest to check) c) also possible but less likely mice have chewed electrical wires.
Check ALL the outlets in the rig, including the patio outlets and any inside your bays to find the secondary GFIC. You can temporaily bypass this outlet or wire in a regular outlet to see if its the problem.
- electrical wiring in older RVs is typical bizarre. In our 1990 Southwind, ALL the circuits are controlled by the GFIC in the bathroom, and once we lost all power because a heavy rain had gotten water in the patio outlet-
The gas side is a little more complex. You will need to check that there is 12v power (which runs the control circuits), so make sure your house batteries are up and/or that the rig is plugged into shore power and your converter is working (do the 12volt lights work?).
Look for your reefer manual here
or google it. There should be a troubleshooting guide for the gas side that you can use. It sound like control board or thermocouple to me, parts for which are still available from these guys
Because gas-absorption reefers are expensive to replace, some people have pulled them out and replaced with small household refrigerators- not a bad choice if you're always going to be plugged in at campgrounds. Not so good if you boondock.