Fair point. I checked with McMaster-Carr and they list all sorts of cabinet type magnets and nicely list their holding power in lb-force. I've decided to get 4 at a 6-8 lb force and start out with two and go from there.
Yes I do think I'll be opening at least once a year to check tightness of power connections. All the shore power and the generator power go through this one box. I'm making a list electrical connection points to check connection tightness. So far, I have the EMS, ATS, and main distribution / breaker panel below the passenger bed. I haven't looked but will probably include the generator connections at the genset. Connections loosening over time due to vibration can cause electrical issues.