Please help. Bunk heater keep beeping

That sounds like the low voltage warning. Get your batteries tested to make sure they're still in good shape. Other than that, you'll have to manage your electricity usage. Charge your accessories while you're driving and idle the truck when you are making food, watching TV, etc. Maybe wear layers and leave the bunk heater off until you're almost ready to go to bed. One thing that I do is to idle the truck one last time before bed so that the engine is a bit warmer in the morning, plus the alternator charges the batteries a bit more. If you keep discharging your batteries to almost empty every night, your batteries are going to fail quicker, and it will only get worse as the temperatures get colder throughout the season.