get one of these. Put it in a dry accessible place behind the dash.
run the 8awg black and red to it.. 8/2 awg duplex will simplify the job.
https://www.amazon.com/Blue-Sea-Syst...ps%2C68&sr=8-3
https://www.amazon.com/Lawrence-Mari...ps%2C55&sr=8-4
The Red wire has to be fused or have a 30A circuit breaker
near the battery.
If you use a surface mount 30A Bussman 185 type breaker it can be used as an on/off switch also.
or... You could use an MRBF battery terminal 30A block fuse