2008 keystone passport 285rl fridge

Nice trailer but you didn't provide ANY info about fridge...brand/model?

Being a 2008 generic info is:
Fridge is electric/propane
Place it in 'Auto' mode..it runs on 120V AC when available
No 120V AC then it swaps to 'Gas' operation

Whether on 120V AC or on 'GAS' fridge needs 12V DC Power for control module, interior light, display panel and propane operation.

Your trailer has Two 20# propane cylinders so if full you will not run out of propane for LONG time. Fridge could operate for weeks on the propane.
Limiting factor will be DC Power.
One 12V Battery........good for maybe 3-4 days until DC Voltage is too low.
Not only fridge DC demand but their are parasitic drains on battery

Taking a wild guess, if you have a full 20 pound bottle it will go at least 3 weeks on propane but you still need a fully charged house battery because it needs 12 volts to operate though the current drain is quite low.

I ran my Dometic on a test run, straight propane, full bottle and house batteries (I have 2) a full month before the bottle ran out. Batteries never did.

To lessen the gas usage. pre cool the unit before leaving on 110 volt shore power and load the fridge with pre-cooled food and the freezer with frozen bottled water, which, you should always do anyway. An ammonia fridge needs pre cooled food as they will struggle to cool down warm items.

Thank you for the input guys, sorry I'll try to get more fridge info tomorrow, all I know for now is that it's a dometic. I don't have any experience with gas fridges, I figured it would need some dc power, just wasn't sure.

My house batt is deep cycle marine, I think about 200 a/h so I may look into adding another in parallel. Yes I do have two bottles but switchover is turn one off then turn other one on but from what you guys are saying I should be ok for short periods.

Looks like some in field trials....ha