Lowes sells a Volt meter with clamp on DC measurements for about $80. They also sell one for $40 that will not measure DC amps. . .
It is brown and black top that clips around one wire at a time, and can measure amperage in DC lines, as well as AC amps.
The Co meter will almost always be on, and it draws a small amount of power all the time. I don't know if it is on a fused line, but it should be.
The propane detector and CO meter in my RV consume 0.8 amps according to my very accurate E-Meter. The refrigerator total amps is 1.2 amps (including the CO and propane detector). So altogether it is about 35 amp hours a day, or what one of my 120 watt solar panels puts out.
Good luck,
Fred.