Had to make the opening about 3/8 of an inch or so taller so the RV550 JUST slid in under a horizontal stringer. The side flanges covered the rest of the hole. The whole thing is in a compartment under my streetside clothes closet. The lid still closes properly. Getting the PEX re-routed was tight, but since this heater DOES NOT need a bypass setup that eliminated a lot of fittings. I did leave a cold water shut-off valve in the supply line just in case.
If you install one use metal duct tape to cover all the seams and slots in the case to prevent air leaks. Do this before you put it in the hole. Don't ask why I know this.
I did have to relocate the gas line a bit to connect it back up. Wiring was very straightforward with butt crimp connectors to reconnect the wires I cut off the old butt connectors. On my AS I needed a custom support block between the floor and the back bottom of the RV550 to than some strain off the skin and flange. This heater weighs a lot less than the Atwood but vibration will flex and eventually break things.
Also, Dear Wife absolutely loves the total lack of restrictions on hot shower time. I like the fact that I'm not carrying 42 pounds of water so far forward on the kitchen side as well. And yes, we do use it on internal water pump and tank. I did see some short-cycling on the pump after the Atwood was removed--no air bubble anymore. I an installing a SurFlow accumulator tank and silencer flex lines at the water pump to fix this (non) issue.
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Almost forgot another small issue: On my AS, the bottom of the heater is between segments of a trim strip around the bottom of the shell. I had to carefully drill out the 1/8" pop rivets that held the ends fittings in place plus back a few inches on each side. I then carefully shortened the trim strip so it would clear the door on the RV550. I used a small wood block to pry the trim outwards to clear the shell and very carefully used a hacksaw to cut it about 3/4" shorter on each side. The wood block protected the shell agains hacksaw slips...
I then re-installed the parts with new rivets and resealed the top of the strip and the rest of that area. I ordered my RV550 with an unpainted, bare aluminum door so it looks nice and matches the color of the AS, as opposed to the ugly painted door on the Atwood. It looks very nice now, and works gangbusters!
'The Silver HamShack' (2007 International 22FB CCD 75th Aniversary model)
Multiple Yaesu Radios inside and many antennae sprouting from roof, ProPride hitch
2012 shortbed crewcab 4x4 Toyota Taco TV with more antennae on it