I would not want to suggest that a certain sized tank would be okay and have it fall through your floor but I can give you some weight estimates of my 75g and 90g tanks.
I suggest, before you install any large tank, you consult a professional structural engineer. I would assume that you would need some support under your floor since I think most of the trailer floors are metal frame with plywood.
Here are some estimates and your values will go up or down depending upon how you load your tank.
A 75g AGA tank weighs around 150lbs dry and a 90g weighs around 170lbs dry. Just the water alone is approximately 8lbs per gallon so a 75g would be near 600lbs and the 90g would be near 720lbs. The standard pine stands for both the 75g and 90g weigh around 50lbs. I had close to 150lbs of rock in my 75g tank so I would guess the total weight was upward of 900lbs when all filled with water and rock.
I know it is a pain in the neck to have someone check out your floor but I would not want to see you loose your tank or you get hurt if your floor collapses.