For me RULE NUMBER one is not to mess with the TRUCK starter battery or the Truck Alternator System. These are only used to get me home.
I leave all of that as factory as I can on my camping trips.
In your case perhaps your 1000WATT INVERTER has two receptacles on it. Perhaps plug the fridge into one and your CPAP in the other one. You can use extension cords if your like...
Not all that familar with the CPAP machines but seems like I hear alot of folks saying they are dual voltgae items 120VAC and 12VDC depending which cord you are using....
You may have to investigate what type of INVERTER is needed for the CPAP machine. I suspect your 1000WATT Inverter is a MSW type and this may not be compatible with the CPAP machine... You need to check on that before plugging it into the 1000WATT Inverter. The PSW type inverters produce the same 120VAC power the Shore Power gives.
A good setup for you would be to pick up a 600WATT PSW Inverter to run your CPAP as well as all of your home entertainment items and other low wattage electronics.. Then just leave the 1000WATT MSW Inverter alone to just support your fridge when needed.
Your dual 6VDC batteries will produce around 220AHs which will hold up pretty good over night.
What you don't want to happen is tie in your truck start battery to the mix and drain both of them down over-night. Then you are DEAD IN THE WATER so to speak...
If you buildup your battery systems you also have to consider how to charge them in a timely manner. You should not run down any deep cycle battery lower than 12.0VDC over night which is approximately 50% charge state and not return them to at least 90% charge state the next day. Doing this will do harm to your batteries and you may have to replace them. You need to come up with a easy way to recharge them when they get below 50% charge state. It takes around three hours time to recharge a deep cycle battery back up to their 90% charge state using smart mode charging techniques...
In my case I must start each day/nights run on my batteries with them charged up to at least 90% charge state. When my battery bank drops to 12.0VDC I will shut them down and wait until I get them re-charged again in a timely manner.
This is what I do at any rate - I'm sure others have their own ideas what works well for them.
It all depends on how you camp off the power grid... I unfortunately consume alot of DC Power from my 255Ah capacity batteries supporting my Ham Radio ops and our Trailer needs...
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS