If your trailer does not have an enclosed belly, all of the frame would be visible. If the shop has a level floor, it is as simple as taking a tape measure and measuring from the floor to each frame rail.
From the description you gave, it sounds to me as though one of the main I beams has cracked, probably near, or at one of the spring perches.
One other possibility is the screws holding the trailer box to the frame have rusted through and the box has shifted.
What you may need to do is determine if, as you say, you hit something, such as a huge pot hole, or curbing, which could have caused an extreme stress to the unit, maybe you will get lucky , as I did and have the insurance company cover it. I had an aluminum frame member break inside the wall of a camper, around a slide. The outer skin had to be removed and the weld repaired and the wall framing re-enforced. The shop also found the left side I beam had lost its upward camber, causing the rear of the unit to hang a little lower on the side where the weld failed. It was covered under comprehensive as an undetermined road hazard collision! All I paid was the deductible. The rest of the bill was 18 K! Might be, your insurance company would prefer to total your trailer and give you a check for the value, which in M. O. O. might be the best thing to accept with a ten year old trailer.
Bandaid