Use mine all the time in the trailer.
First, with a couple of 6V golf cart batteries you can go a long time with a CPAP especially if your CPAP has a 12V input. If not you'll need a small inverter.
Second, Here is the BEST choice for dry camping. Check this website for a CPAP battery and nice small inverter if needed. It will run a CPAP for multiple days on one charge, can be recharged from your car or trailer battery, is compact and in my case I was able to buy it with health care reimbursement pre tax $ since it is considered a medical accessory. IIRC it resmed CPAP's must use the inverter, not 12V, but mine is a REMSTAR and it works fine.
link:
http://www.batterypowersolutions.net/
I have the C-222 battery pack and small inverter.