A. Smith,
To best answer your questions, we need more information about your current battery, charging and loads. It seems that you have a single Group 24 AGM battery that's lasted you for 6 years (that's really good). The "controller" you mention is probably a 120 volt AC to 12 volt DC converter that powers your lights and other DC loads - and charges the battery - when you're plugged in to 120 volt AC shore power.
You mention your CPAP. It would be helpful to know if you're only powering it when plugged into 120V AC shore power. Or if you're "boondocking" without shore power are consequently are running the CPAP directly off the battery (12 volts DC) or using a DC to AC inverter to power the CPAP with 120V AC.
I use a CPAP machine that will run on 12 volts DC using an adapter cord sold by the CPAP manufacturer. (Many CPAP machine power supplies used at home just convert 120V AC to 12V DC before connecting to the machine. You'll save money and battery charge in that situation by using the 12V adapter to run directly off your battery.) I generally don't use the CPAP humidifier when running on battery power because the humidifier uses a heated plate that uses even more electricity than does the blower fan in the main CPAP unit.
I bought a Group 27 battery (12.0625 x 6.8125 x 8.875 inches nominal size) that fits fine under my Hawk front seat. It's rated at 110 amp-hours which is a bit of a boost from your typical Group 24 battery (75-80 amp-hours). I made sure to get a true deep-cycle battery and not one of those marine deep cycle batteries that is sold to boaters who need something to both start their boat engine and run their electronics. Expect to spend about $300 on a name-brand Group 27 AGM deep-cycle battery.
You don't want to regularly use more than 50% of an AGM battery's capacity before recharging it if you don't want to shorten its life. In my case, 50% of a 110 amp-hour battery is a useful capacity of about 55 amp hours. My CPAP draws 10-12 amp hours per night. So I can get 3 nights of CPAP use (plus all the other lighting and cell phone charging, but no furnace) without dropping below 50%.
So, tell us a bit more about your system and we may be able to give you better-tailored advice and information.
Ed