Shell Idealization Over the Solid Model in Geometric Element Analysis

Hello,
In relation to the above query, here is a detailed explanation of what I am trying to do with this FEA study. Basically, I am trying to analyze a scenario where a genset is placed onto a trailer and when in transit, how the Genset will move when sudden braking of trailer happens. For a better understanding, please see below a pic showing the model where genset has been placed onto the trailer:

When preparing the model for FEA, the "review geometry" function for trailer is working fine where it shows the bonded surfaces (default connection kept as bonded). However, for the genset model, I am having following errors when trying to use "review geometry": 

Detected bonded interfaces between several mated solid surfaces.
Check your model for volume interference and/or add a contact interface.

Source: Geometry

I have rechecked for any components or surfaces overlapping for the genset assembly but could not find no more and I am still having the same issue. I am not sure what it actually means or how to resolve it. Please help me understand and solve this problem. How and when to use contact interfaces?
Please see attached here with the zip file for the model incase you want to give it a try. Any input will be highly appreciated.

Thanks 🙂