Within the stood-up/turned-out FFV positions in the Offroad truck that allow firing of Secondary weapons (RPG, Titan, PCML etc.) it is not possible to use the 'Switch to Secondary Weapon' keybind from the controls, to switch from your primary weapon (rifle etc.) to the afore mentioned secondary weapons.
The older 'Switch gun / launcher' keybind can be used to shoulder the launcher, or by selecting it through the action menu.
The 'Switch to Primary Weapon' and 'Switch to Handgun' keybinds work fine in all FFV seats alongside with the older 'Switch gun / handgun', as does switching to and from binoculars etc. It is only the controls for the launcher slot that are not consistent with how they work when on foot.