Depends... Normally I put the jack in the large basement compartment just aft of the driver's side rear tire.
But I use the jack for multiple jobs..
Since the hydraulics (Leveling jacks) on the RV have issues, I put it under the frame, any handy support point such as the spot where the springs attach to the frame or the torsion bar (Sway bar) using blocks to lift the jack so I can jack it up and place a jack stand.. 12 ton air/hydraulic jack makes it easy
For tire changes, under the spring shackle. But.. One can not always get a jack in there (with 22.5" tires you generally can) but that is the support point for axle lift.
I also use frame support points when I have to.
On the car, bit more complex, flat nothing fits. So I jack, block, jack, block, and finally get it under the spring shackle.