First I will add the disclaimer on the rear bumper. most rv's have the square on back used for storing the dump tube and a spare tire. their not a "bumper" and cannot carry more than 150lbs max (mine). I have a 2021 Winnebago 2108DS micro minne. it came with the spare mounted on the rear square.
my holding tanks are 25 gal, I use a 36 gal camco portable waste tank because I did not want any possibility of overfilling one when I need to use it as well as the bigger one comes with 4 wheels and a strong tow bar.
I wanted to carry the tank on the rear and made some modifications I thought I would post to possibly help another camper. the tank is great, I modified the wheels before ever even using it to steel centers and solid rubber tires. drilled and cotterpinned on.
I bought the camco rear mount arms. and as is, these are much to flimsy to be worry free having it back there. I welded 5 more 3/4" sqaure tubes set apart per the distance required for my size tank, then some side gusset brackets at the arms radius, and lastly tack welded the u bolts to the square tube, and tack welded the arms to the top of the rear "bumper". its super strong and worry free to have back there.
I wanted less weight on the rear not more. so I removed the spare tire bracket and spare and used the b&l under mount cable assy and moved the spare up front due to my 2108DS has a perfect recessed cavity up there to tuck the tire in to. didnt want to really but I did mount it with access from the drivers side because my gas line was in the way on the passenger side and I did not want to hassle with new line and re-routing it. my luck is such when I have a flat its no doubt on that side anyway lol.
I now have a lighter weight on the rear than the spare was,
and the big 36 gal tank no longer takes the space in my truck bed!