Variable Fan Controller - Properly Sized Transistor
Doing a quick search for high-current MOSFET transistors, I ran across the following:
https://www.digikey.com/en/products/detail/ixys/IXFK170N20T/2126313
The important spec here is the Rds ON (max) of 11mOhm @ 60A, 10V
At your peak current of 100A, you would drop 1.1V across the transistor, for a power dissipation of 110W.
The advertised P(max) is 1150W, so assuming the junction temp can be held to under 175C (and with an entire metal truck to use for a heat sink, this shouldn't be a problem) you should be OK with this device.  In the 'cruise' configuration with 25A motor current, you should have no problem controlling the fans.
YMMV ;-)