I understand the battery cutoff switch, I have one on my truck. I used the biggest amperage switch I could find, 2500 peak amps, 250 continuous amps. Yours is rated for 135 amps, thats why I suggested removing it till you solve the heat problem.
While waiting for the rain to pass, pull the battery and have it load checked, do it three times to simulate real use. If it still good the third time, you know it's not the battery.
