I think you could, and at a reasonable cost. But it "depends". An engineer's thoughts:
1. If it is aluminum sided with wood studs, it may or may not be difficult to re-enforce the opening with new framing material.
2. If aluminum sided with batt insulation, you may run into wiring that has to be dealt with.
3. You'll need new entry steps and you will need to see if there is no propane piping, stab. jacks, slideout parts, etc. that would make installing new stairs complicated.
4. If a laminated wall with fiberglass siding, maybe. Need to find out if it only has aluminum framing in the corners or studs down each side. If no aluminum studs, you can easily cut an opening.
5. You'll need to be handy and have some tools. Paying a shop to do this could cost thousands. You'll never recoup the cost when it eventually gets sold.
6. Depends if where the 2nd door would be is a practical location, or you'd be "forcing" it to work in a less than suitable location.
7. You need to consider if it's worth the time & expense (as mentioned above). Depends on if you are in love with your TT, maybe have done a lot of other upgrades/mods to it, what the replacement cost would be for a new unit, is it an older TT that could have rot and/or mold hidden in the walls, etc.
Many TTs have 2 entry doors on the curb side. Ours does. Our manuacturer (KZ) has a lower level model with the same floor plan with just one door. Our walls don't have studs down each side.
A fiberglass sided TT provides "some" additionally rigidity to the overall structure as the wall assembly acts like a diaphragm. If you happen to have a trailer that has the type of I-beams that are made from 3 pieces of mild steel welded together, these I-beams flex quite a bit more and it *could* be possible that the superstructure sitting on top of the main frame/I-beams is weakened enough to cause problems such as broken welds in aluminum framing.
What is the make and model of TT and perhaps a photo of the wall and a floorplan might help.
This is something I would tackle providing I'd have looked into the above, and maybe anything else I haven't thought of at the moment. Depending on the who the manufacturer is, you may be able to get some comments and advice from the factory. I've done a number of complex mods to our TT (that nobody else has done out there) and this isn't any worse at first glance. Maybe ya'll should drop by here one weekend and we'll slap it in. :)