Might be under the passenger seat. There's supposed to be some battery connection/separation wiring and parts under there, based on the threads by people who add the Trik-L-Start chargers to their coach/chassis battery charging configuration.
Just a guess.
All comments are my opinions, and worth exactly what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????) - 2016 Sunstar 26HE (sold @ 4600 miles) - 2002 Roadtrek C190P (sold @ 315,000kms)