To install Progressive Dynamics EMS 30 Amp "hardwire" device, I would simply remove the factory galvanized junction box and replace with EMS unit. And, apply fasteners to secure EMS to floor and wire fastener hold downs < 8" from the EMS box as well. A really good video at:
https://www.youtube.com/watch?v=6CSg2ryqE7UWithin YouTube, I'm sure other good videos under same "RV Surge Protector Install" text as well.
Note: If uncomfortable with electrical wiring, do ask a certified electrician to install instead. Very easy - especially since you can tow the job to them.
Hope this helps.