I put a truck imported from blender into a scene with some other vehicles. As soon as I assigned a rigibody to this truck it started falling through the terrain floor, which stops if I tick kinematic or untick gravity. None of the other vehicles have this problem even though they also have the same rigibodies with the same settings as the one for the truck. Thanks
Two things:
- Make sure that your ground is kinematic and rigid as well.
- In blender, you need to make sure that your mesh is not offset from it’s pivot. This will cause any colliders you attach to your mesh to not line up with the geometry; often times bisecting the “floor” and causing the solver to fail.
I guess one last thing to check is to make sure you’ve got colliders on everything but that seems pretty obvious.
Your truck has no colliders by default. You have to add a collider AND an rigidbody. Adding only Rigidbody will have it fall through, because it has no collisions 
Since your truck is probably a skinned mesh, you can’t use convex colliders (i.e the trucks mesh as collider). So you should use multiple primitive (box, sphere, capsule) colliders to cover up your truck.
edit:
Same goes for the ground. Since the ground is static, it’s ok to use convex/mesh collider.
Thanks Tseng, a box collider works great! 
In my case, it was the terrain. I just started trying things until I hit upon the answer. In the terrain collider, I unchecked the Is Trigger checkbox, and my imported Blender jeep now does not fall through the terrain.