My GF owns a 70's double wide Mobile home that she rents out. The last renters trashed the place and on top of that never bothered to mention the leaking shower. I am in the middle of spending all my spare time refurbishing it. There was NO structural integrity left under the shower and the wall the shower was on. I literally fell through the floor where it had gotten wet.
The subfloor is 5/8" PARTICLE board. The 2 x 6 joists are 12" on center. I wouldn't worry about the weight of a 55g or 40g. Much more than that and I wouldn't do it. My biggest concern would be the inevitable water spills soaking into the particle board floor. With a bit of water on particle board it won't hold a 5 gallon tank. I would figure out how to do some kind of vinyl flooring or tile containment pad for under the stand to catch all the inevitable little spills.
We will never rent to pet owners again. We will not even allow aquariums. The repair costs are more than the accumulated 3 years of rent the previous renter paid... who has since disappeared.
The guy I bought my 110gallon tank from had it along the exterior wall in the mobile home he rented. The mobile home had relatively new laminate in it. The laminate around the tank was all cupped and puffed up at the joints... ruined! When I noticed it he commented "improper installation"... yeah right! He seemed like a pretty decent fellow... but people just don't seem to express much caution when they are renting someone else's dwelling.
As a renter I get along very well w/ my landlady... we're honest with her about our tanks.. and we've been okayed up to a 75 in our appartment.. though I'm fairly sure the flooring could hold more. You never know what potential issues you might have with the flooring ect.. if you don't ask first... as much as I love my pets.. a roof over my husbands and I's head is more important.
This was also one of my first questions when looking at the appartment. which may end up good or bad.. you may have to look longer.. but in the long-run you'll do better as a renter always asking about aquariums/pets. I've known a few peopel evicted because they refused to respect being a renter.