The M810 was actually a chassis/cab, the short 167" wheelbase version, so you could put what you wanted on the back.
The M817 dump truck and M818 tractor were "standard" vehicles on that chassis, but I have heard of generators and such being put on there.
Opcom recently built basically what you are looking for, an M109 box on an M818; it is well executed and super cool!
I don't know what box is on the back of the one in your picture. I figured that was maybe a Seabee or a beachmaster truck. I can't make out the emblem on the doors. Looks like sand mats on the roof? Whatever it is, I like it!
Cheers
ps- don't remember where I got the pictures, so I can't give credit for them at the moment...
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