With a relatively light load for a truck’s capability, COG doesn’t matter. Supporting the floor is the issue.
There’s no rule about having the COG in front of the axle. In fact for many trucks, diesels in particular, you don’t want to add any weight to the front axle. GM’s camper loading literature (at least older stuff, last time I looked) lets you put the COG all the way at the rear of the bed, so long as a certain minimum percentage of total weight is carried by the front axle (something like 25-30 pct.). I doubt the physics change for the other manufacturers.