From the previous test of loading through "popup of vicinity", i noticed that it displayed the percentage of space used for the trunk.
Cannot load further 44% on M998 light utility for example (haven't tested all the vehicles yet, will update with what i find later), further than that sometimes you will reach fewer space usage depending on what you're trying to load.
Example : I can load 2 AT LAW, but only 1 M249 ammo clip (seems to me the clip even from M249 take way less space than 2 LAW).. so there is also discrepancies depending on what ammo type you're loading.

US M151A2 Offroad : 27/28% (tried with clip & LAW)
US M823A1 Truck : 15/27% (stuck on 15 by loading handgun & sniper clip only ; 27 by loading a AT LAW)
US M923A1 Transport Truck : 100% !!!
US M151A2 Armed Off-road M2 : 48% (clip & LAW)
US M934A1 Command Truck : 52% (2 LAW) or 16% (one 200 round M249 belt) or 22% (one M249 belt + 3 5.56 mag)

I guess i could try all the others, but you're gonna figure out the bug affecting mostly all US vehicles (transport truck seems fine, i tested all belt even the M2 that fits very rarely in others vehicles)

Still have to check few RU vehicles later.
Haven't tried with equipment & weapons (other than LAW) yet. But maybe you can test that on your side also ?