Walt,
On my TS the water heater is contained in a compartment that actually could be configured as a pass-thru. On the driver's side it is for the WH. On the passenger side it is a storage compartment with the pass-thru blocked by a cover. Behind that cover in the pass-thru (over the frame rails) there isn't anything except the cold and hot water pipes leading to the heater. The area of this compartment for the WH was about 20" wide inside, and about 27" deep disregarding the pass-thru. It was probably 24" high or so. The actual opening to that compartment (behind the outer skin) was about 18" wide. Plenty large enough (in my unit) to accommodate any Atwood or Suburban.
It would have taken less work to adapt a 6-gallon unit, but I wanted more capacity. So after carefully researching heaters I decided on the 10-gallon XT. It has the electric element too so I could heat my water on campground power when available. something the tankless unit was lacking.
Attached is a link to the Mods forum where I described the install. It also has a picture of the original opening (after the Girard removed) so you can somewhat see how large it is inside. Here is the link:
http://www.jaycoowners.com/forums/f3...7ts-31841.html