My friend lives in an older trailer. The subfloor was plywood and the framing is comprised of huge steel I-beams underneath. It's really easy to peel back the skirting and check it out underneath. I'd imagine it would be relatively easy to do some structural modifications under there. I think a lot depends on where you live. Up north here the trailers sit on footings that are 4 feet underground to get below the frost line. If you add a structutal support and it isn't properly anchored you may end up having it heave upwards during a freeze, causing catastrophic damage.
If it were my trailer, and i were single, i would peel back the carpet and subfloor and weld in some heavy guage square tube stock to bridge the gap between the Ibeams.
Of course if the tank happens to be sitting directly over a structural beam you may be able to get away with it assuming the subfloor is in good shape.
I'd consult a contractor. I've found that actual contractors have a better knowledge of structural elements in homes than some engineers or inspectors. I work around engineers almost every day, more times than not, the guy with the high school diploma on the floor has a better understanding of what does and doesn't work.