RVI3 owner here... I ran into the same problem when running the breakaway wire for my Chevy Silverado... I gave up going through the firewall, instead, ran the wire through the driver side fender from the engine compartment. From there I ran the wire through the rubber boot where all the door wiring is fed into the passenger compartment. I just cut a small slit in the base of the boot where it attaches to the body.
Also, not sure if you're aware, but RVI has just introduced a wireless version of the breakaway switch. Not sure if it would work on the RVI2 or not...
Duane & Claudine Miller
2020 Fleetwood Discovery LXE 40M