That bug exists since ages. The M916 is not exactly the most versatile and powerful truck with an overwhelming range and stability - but its too limited in even with semitrailers.
The sideboard has a collision issue as soon as its "bending" a bit in a slope, the semi-fueltanker cant even be mounted ....
(a) What about either having that dammn winch on its back a purpose (its currently rank restricted to 20) - e.g. giving that winch a single "The Winch" option with long range and no need to upgrade. Really long, really powerful. So that truck has a really strong "pro" if something is stuck somewhere.
(b) resolve that issue by removing that winch at all - with the option to mount an "utility rack" (see F750) having some repair, tires and fuel. Currently, the winch looks nice but prevents proper truck usage with half the remaining semitrailer options having issues.
Regarding stepdeck: Why not round the edges a bit off and allow the stepdeck a single cargo slot at the front so it can carry 5+1, 2+2+1+1 but not a 2+2+2 Cargo? Or put some spare tires as well as some "utilities" (repair & spare fuel) on that unused space?
Making a "gooseneck semi" from it with 5 slots is pointless, that trailer type exists. Maybe as a saddle low offroad gooseneck with 4-5 slots so there is more vertical movement possible and still no collision with the M916 or similar haulers? The current "gooseneck offroad high saddle" with only 3 slots is a joke with respect to weight/size and capacity. It looks good, but makes no point in gameplay.
Edit: If you want to make the M916 "unique", you may also think of giving it a lower center of gravity and this addon:
Some spare parts, a bit of fuel and there is your revovery crane. With a really strong winch, but not that much crane range. Even if the turning of the crane would be restricted to the rear (front side being blocked), that would be OK. Suddenly there is something special and a truck having a purpose in gameplay.