To start with, wendywoods -- welcome to the forums!
The 27 & 28 are two very different Airstreams. The 28 is 27'11" long, has a front door and rear bed. The 27 is 28'0" long and, being an FB model, has a rear door and front bed. (Go figure, eh?
Colonial Airstream in New Jersey has some of the better photo documentation of new models. The 28' International
has a rear storage compartment under the head of the bed (nice). The HW heater is on roadside, below the L-Lounge just in front of the sink and directly under the front side window. I assume the Parallax converter is somewhere under the L-Lounge also.
The 27' International
has a front storage compartment under the head of the bed (again, nice...). The HW heater is streetside below the bathroom cabinets or pantry. The Parallax converter probably is below the base of the front queen bed (shouldn't be an issue).
Figure that the black tank will be directly under the toilet (very different location in each of these two models!). The fresh water tank would be at the other end of the three probably.
Now... "our" local dealer has many diverse interests, of which Airstreams are a small part of what they sell. I've found that my research easily exceeds their knowledge base. There is no way that any water tank accessories or waste tank vents are anywhere above the floor for either the rear bed 28 or the front bed 27!
2airishuman has given good advice. The factory needs to be consulted about what they will do as a custom job. There will be a $500 change fee if they do this. Flooring needs a minor change. The Parallax reposition is virtually the same that they do for the 25' FB Queen vs. Twin.
Definitely see if you can get the roadside & curbside compartments under the twins -- my 25' FB twin
has these 2nd & 3rd compartments. They are very handy! The compartment doors match the inter-rib distance, so no ribs need to be modified.
The 27 & 28 have extra length in the bed area of the trailer -- more length in that area than the 25' FB has. So you might fit in extra wardrobe space or a stack of drawers on each side or something.
If the factory wouldn't do this, I'd try some more experienced dealers. In order of increasing distances, I might check out Airstream of Arkansas, [ no - Roger Williams in Texas is no longer selling Airstreams ]. Maybe Out of Doors Mart in North Carolina. Furthest away is Inland in Corona, CA. US Adventure RV in Davenport, IA, seems to have strived to maximize their in-shop offerings for Airstream owners -- I don't know if they have experience with this.
Let us know what you find out!