I don't have a Seneca but a Greyhawk MV (class C).
I just completed a building to house the MV. I went with two roll up doors to accommodate storage of stuff as well.
The build is 28' wide, 38' long, 16' tall, two doors 12' wide for the coach and 10' wide for other stuff. The coach is officially 32' long.
The coach door is a 12' roll up door because the MV is 8'-4" wide.
I placed the coach dead in the middle of the building door to assure the mirrors will clear equally. Then painted a high-vis orange strip on the cement floor along the lower edge of the coach on the driver's side(easily seen from the rearview mirror).
Following the line on the floor (by looking in the rearview mirror on the driver's side) assures me that both mirrors have ample clearance to back the coach into the building without threat of hitting the mirrors on the building.
I then placed the coach into the building with 2' of clearance from the wall to the rear wall of the coach. I placed an 8'x 4"x4" cut in half behind the rear tires, anchored into the cement floor to stop the coach from hitting the wall.
This setup allows me to back the coach into the building unassisted (if need be).
With the coach in the building (as a test) I opened the slides (on the driver's side, no slides on the passenger's side) and noticed there would ample room if the build was only 17' wide.
I hope this helps you in some way.