This is going to get back to where you are pulling power and wire sizing serving it up.

I'm guessing you have a cutoff switch. If you have a wire headed forward to power the cabin, what is sizing? Maybe take a secondary wire and piggyback it forward to see if voltage increases to a higher level when load from accessories increases. If the voltage is higher, you need to upsize or piggyback another wire forward would be my guess.

Yes on the cut off switch and NAPA made me a big dog years ago but not sure on the exact size but it was WAY lighter than the welding cable and I only lose about a .10 of a volt from the alt. to the batteries in the trunk...........The HHR fan and the Moroso water pump are needy for sure in the amp dept...........