I went through this exact process two years ago. The Whirlpool is a dream with almost twice the capacity of the Norcold LRIM1200. (A well documented fire hazard) Mine was installed by Beckley's in Thurmont, MD. Keith spent a good hour measuring the opening and available space BEHIND the Norcold. Because of the burner tube and various other junk on the Norcold you will gain almost three inches in the rear. Consequently, the Whirlpool will fit nicely. In the case of my '01 Monaco Exec there was no propane heater under the Norcold and the Whirlpool was able to side almost on the floor of the coach. Berckley's build a "stage" for it and it is nicely secured. I did lose the cabinet that was located on top of the Norcold but it was finished nicely by the cabinet shop at Beckley's.
The old Norcold and the new Whirlpool came and went through the front windshield...or maybe a side window. I wasn't there for that part.
I do have a set of pictures of the installation if anyone cares to see them.