I understand your concern….
My comfort level is that something that heavy should also be able to carry my own weight and momentum, if I ever fell on top of it in rough seas
Apart from being heavy, they go thru a lot of temperature changes (especially isolation transformer) if working hard and are very expensive.
My Victron units, (isolation transformer and multi system) come with a “wedge plate” for mounting on wall then hanging unit on, with 2 additional fasteners at base. (7 total)
I did not trust just that alone going into wood and since they want maximum cooling
, I made a heavy open box 3” aluminum
standoff to keep unit well away from bulkhead
I then used an alloy backing plate behind the plywood to mount the box with machine screws and stainless wood screws into additional hard timber
If you need to double up on plywood to get the backing plate and you have no internal studs, then you can also fashion a timber cleat to catch under the box so you are screwing both into bulkhead and down into cleat so that the cleat is also taking the weight as well
The wedge plate is thru bolted with back lock nuts onto box plate.