Tires/Water tank.

Just a side note...The bulging tank underneath may be a sign of impending doom. I noticed mine starting to seem lower when full, and going on intuition alone, I removed the plastic underbelly to bring into full view the fact that the tank mounting "system" they used was nothing more than a piece of 1/2" OSB between two bar joist flanges underneath the floor. The condensation over the 8 years had gathered at the center of the tank and caused the OSB to rot right in the middle. There was precious little supporting my 400lbs of water. When I took the OSB out, it was visibly rotted and I was able to break it in half with my hands. I replaced it with 5/8" plywood with steel straps underneath and some weep holes to allow water to drain and evaporate, and some plastic strips between the tank and plywood to allow air circulation. No more sag. You may want to check yours.