Jammer is correct!
Atwood water heaters come with an integrated flange that bends over the outer wall of the RV and is screwed in place to (after the inside wall of the flange is lined with butyl tape).
Suburban water heaters have no such integrated flange. You have to place the heater where you want it and screw it into a plywood floor or retaining block. The flange is part of the outer door assembly and attaches to the body of the water heater by 4 screws; one at each corner of the flange. Butyl tape is also recommended to be places behind the door flange.
If the Suburban heater that you ordered fits into your space in the sidewall of the trailer and you want to use it, you will need to order the appropriate door and flange assembly. If the dealer you bought it from has no idea what you are talking about, perhaps you should find a dealer that is more familiar with the products that you want to use.