First, you need to know the power requirements of your particular unit. Many now are 24 V DC in. You can go to CPAP.com and look up your unit to see their offerings and likely get this figured out.
Then you need to look at the trailer and see what it has available. Third, you need to consider your use: Boondocking, Generator, solar, shore power?
Generally, with shore power and an available 120V outlet, just plug it in like you do in a home.
Coming through the unit's batteries there are myriad options. The cleanest is likely to use a 12V Cig lighter type receptacle if one is available. Then if you are 12V you can plug right in with a proper cord (order a cord if needed), or if you are 24 V a purpose built step up transformer is what you want, something like this, but configured for your machine:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It's always good to have a Li-Ion backup battery for a couple of nights, like the Freedom suggested above. I use this, with more ah than the Freedom. But, the price has been skyrocketing...
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1 As noted above, turn off water and tube heat when on back-up battery, or whenever power is limited. Passive humidification is usually adequate.
Another option for running off of batteries is to install (or not install, but use loose) an inverter and a 120V receptacle. Most CPAPs require pure sine, but you don't need a huge amount of watts. 150W is probably more than adequate, or even less. Again, know your machine. In this case you simply plug into the inverter itself, or your installed receptacle as you do at home.
Good luck!
For 24V, you can also run off two 12V batteries hooked in series.