While I would not recommend the practice to others, I have been hauling two bikes on a spare-mounted rack on a Bigfoot
17' for 70k miles. This has not been without incident.
Note that my Bigfoot has the springs sitting atop the axle. This significantly raises the body, giving more ground clearance. I would not consider a spare-mount bike rack otherwise.
Before driving with the bikes I had the spare tire mount [not the rack] reinforced. On either side of the mount, at the top, tubing was welded at a 45 degree angle down to the steel bumper. Steel strap was placed from the top of the mount forward and down around the bumper, then welded in place. This can be seen in one of the attached photos. I thought this would be enough. It was not.
The spare tire mount has a 3" or 4" long steel crosspiece with a hole in it. The hole is to accept the bolt that attaches the wheel. That piece broke after several months on the road, including rough and unpaved roads. The bikes and spare seemed secure, just tilted down against the bumper. I had a new heavier piece welded in place. As part of my walk-around I monitor the distance between the bottom of the spare and the bumper.
The big scare came a couple of years later. During a walk around, my practice at every stop, I noticed the bikes and spare were tilted back, away from the body. The welds holding the bumper to the frame were fine. The bumber metal itself had torn on both sides. I had triangles of steel plate welded at the bumper-frame interfaces. That was about four years ago. I keep a close eye on that. No problem so far.
I use the spare-mounted rack to save weight. I admit that I would like to get a receiver hitch and tray mounts. This may need require a custom s-shaped mount tube in order to get the trays up high enough that they will not bottom on steep driveways and to extend far back enough to clear the spare.