Giedo's points are well made.
I would vote to have a window mount (attached to the roof railing) along with a gimbal head AND a bean bags as well. Bean bags cost next to nothing and add no bulk or weight when travelling (buying beans locally of course).
Mounts are expensive and bulky, but here's why I think they're important for safari shooting.
A large lens, like a 500mm F/4 or bigger, is heavy. Even when resting on a beanbag it needs to be balanced. Effective wildlife shooting can mean patience, sometimes waiting hours for the right moment. After 5 minutes you'll tire of standing, supporting the camera and lens on the beanbag. You'll then put it on your lap or on the floor of the Landrover. Then, when the moment for the shot happens unexpectedly (which it will), you'll miss the shot.
With the camera and lens mounted on a gimbal you can relax, watch the scene though binoculars, and then be able to take the shot in a moment when the situation happens.
As for being on the wrong side of the vehicle, well. it takes just 10 seconds to release the mounting clamps and move it to the other side. This can be done while the driver is positioning the vehicle.
Bottom line, if you have a large heavy lens, then a mount system could make the difference between getting the shot or not, and between being completely exhausted at the end of the day and only semi-exhausted.