I took a different approach, I installed a secondary 100AH solar AGM battery into my truck under the back seat for accessories (light bar, accessory charging, CB radio etc) and put a renology 40amp DC-to-DC charger to separate the secondary battery from the primary charge circuit for 2 reasons. 1) keep the primary system only for starting the truck and 2) a solar battery will have a higher float charge and a vehicle alternator won't fully charge it. I can run my CPAP 8+ hours without concern and in the morning I'm usually off and away and the charge controller tops off the battery. Down side is I need to camp close to my rig and need a couple 12v extension cables and it wasn't exactly cheap. Alternatively you can swap out your car battery for an optima Yellowtop but if you do that confirm that your draw for your cpap per hour is at or below half the rated battery capacity divided by however long you intend to run the CPAP. IE if the battery has 55AH, you'll want to make sure your CPAP doesn't pull more than 3.5amps an hour assuming you'll be using it for 8 hours (55/2/8=3.4375). You should still have enough energy to start your rig in the morning.