Resmed and Respironics seem to be the most common brands. One operates on 24vdc and the other operates on 12vdc. I don't remember which is which. Both have cords available for plugging into the 12vdc power in your RV.
When shopping for a C-pap we got the 12vdc brand. The extra cord for plugging into 12vdc was $25. The 24vdc brand requires a transformer which increases the cost of the cord.