I had the 5th wheel dealer install my fifth wheel hitch as a deal maker in my 6.5 foot truck bed... No cost to me...
Your fifth wheel hitch will mount just slightly ahead of the rear wheels in the truck bed. Plenty of room for tool box behind rear window.
I also installed myself one of those FOLF-A-COVER bed cover that has the folding hard plastic lockable covers...
I was going to setup two tool boxes like this google photo but never did do it... I just have a slide around large plastic "ACTION PACKER" tote box with lid (LOWES) at the present time.
The two tool boxes would be a nice setup to go along with the folding bed panel. All the panels fold up behind the rear window when using the 5th wheel and all locks everything out of sight when you are setup at the camp ground. When not pulling the 5th wheel you gain access to the front tool box by opening the front bed cover. The rear box by dropping the tail gate. I can also open the front box when hooked up to the fifth heel but it take a little effort keeping the folding panel all folded up vertical. Can be done...
My 2K Honda Generator lives secured in the tail gate corner of the truck bed. I like to keep it covered up for security purposes when not towing the trailer. The old "OUT OF SIGHT-OUT OF MIND" thing...
ALL PHOTOs from GOOGLE IMAGEs
This is a shot of my 2KW Honda sitting under the fold-a-covers when pulling my OFF-ROAD POPUP trailer. This locations does not restrict the use of a 5th wheel hitch on either tailgate side location.
Roy Ken
Food for thought only here...
Roy Ken