Inverter ?

just watch your voltage running what you would normally run. when it drops to 12 volts, fire it up, and also remember hopw long it took to drain the batteries to that point. the more you ask of it (microwave, etc) the faster it will draw it down if you're parked

 

My inverter would peep that is when I knew it either needed to be shut off or the truck started

 

when installing your new, bigger inverter, dont forget to put an inline fuse or circuit breaker between the batteries and your inverter. I like the circuit breaker because you can push a button to reset it instead of replacing a fuse. Also, if I use my microwave, I start the truck. Then there's no risk of falling too low on volts midway through cooking dinner. good luck

 

Outside temps will play into that as well. I had one truck that I couldn't run my bunkheater for more than 5 hours, due to the batts being weak.