I have this EXTRA SUPPORT MOUNT installed on my Shamrock 183 bumper. I have not only a bumper carrier rack but also a bike rack mounted on the bumper. Yes
I ignored the bumper warning sticker, so no need to remind me the bumper cannot handle the load
My camper came with a bumper mount spare (the dealer ignored the warning sticker too!), which I removed and carry in my TV, and I replaced that with an even lighter load, spaced evenly left and right nearer the weld points of the bumper to the frame. I then added the 4square mounts that you mention. Make sure you have good large size drill bits to make it thru the frame. I C-clamped the bracket into position, then used the holes in the bracket as guides when drilling the frame. I feel they can only help and not harm the strength of the bumper.
I carry my leveling boards and two folded camp rug mats in the aluminum cargo basket. These most often get dirty, wet or sandy and I did not want to put them in the trailer or TV. The carrier is made of aluminum so is light weight and I don't load it with firewood or anything heavy
. The bike rack is on the right side of the bumper and it is not a lever action Swagman-type
with bikes hanging by their top frames bouncing with every bump in the road. I custom made out of aluminum square tube, a rack that supports the wheels of my bike and is with-in 4 inches of being above the bumper. It does not hang out over the rear end very far so lever action is almost non-existent as is the bounce so common with other carriers. Both of these carriers are attached using two of these...
Curt RV Bumper 2" Trailer Hitch Receiver Curt Specialty Trailer Hitches E-100
I know there are some on this board that will ridicule me for doing this, but I have thought out the bumper loading and am confident, so far, that the bumper can handle MY loads. My trailer is less than 20 feet long, so the up/down bounce on the ends are not as severe as a longer trailer... THINK teeter-totter. I do check my racks at every stop, which is usually every 2 hours and more often then that if I have been driving on bumpy roads. I have not seen any bumper weld fatigue in about 4,500 miles of travel so far in 13 months of ownership.
If you do add a carrier, think about getting the light weight aluminum carriers (Harbor Freight) and instead of centering the carrier push it to one side or the other where the bumper frame connections have the most strength. I also pushed the mounting tube as close to the bumper as possible and drilled a new hole into the mounting tube to keep lever action to a minimum. And do NOT over load the carrier, which is really asking for trouble.
If you are one of the ones that do not think this should be done, then pass me on the interstate please and stay out of harm's way to the rear of my Shamrock.
Finally, I have given up on firewood because of the insects, generally poor quality available at campgrounds, and the high cost. Instead I spent about $80 on a portable propane-fired firepit similar to this.
I bought mine at Sams about 3 years ago. It serves me well when I want to roast a weenie or burger over an open fire. I have a camp cooking basket that will hold steaks, chops, burgers or dogs and even shrimp. Something like this:
Amazon.com: Rome's Original Basket Broiler (9" x 12"/Chrome): Kitchen & Dining
I know many of the FL State campgrounds sell firewood as a fund-raiser. I support that and will pay
for a bundle but I leave it in the rack for someone else to purchase and burn. I have gotten out of the habit of having a fire after having been offended a number of times by a neighboring stinky unattended fire that blows into my campground space and I have to put up with the ashes and smell from someone else's fire.
I hope this long reply helps the OP out and gives other campers that have asked about adding carrier's to the back of their camper some food for thought before dismissing the idea or going ahead and adding one.