The correct answer is, "It depends." What do you want to accomplish with it, and how much do you want to spend. Precision Temp, Truma, and Atwood (PrecisionTemp licensee) all make a version that works.
I have a PrecisionTemp 500 series unit. Fits in almost the exact same hole as a 6-gallon Atwood. I had to make the opening 3/8 of an inch taller to get the unit to slide in. Although the unit itself is quite narrow, the enclosure has wide flanges left and right to cover the Atwood hole.
Was not particularly difficult to install, took a couple hours at my usual slow pace. It uses the same plumbing and electrical as the usual Atwood, but I had to relocate the original gas line a couple inches and bend it differently to make it connect properly. I did seal all the gaps and seams on the edges of the enclosure with metal duct tape to prevent any air leaks into the interior of my AS.
I did talk to the factory and ordered one with a unpainted aluminum door. Normally they are painted white, like the old Atwood. Did NOT like the look of white panel against all the shiny aluminum. It looks very nice, and is a similar color to the rest of the Airstream.
I did have to take about 1/2 inch out of both ends of the rub rail molding on either side of the water heater door and refasten it so the door would close properly, but that was just some very careful work with a hacksaw and a couple wood blocks and some blue painter's tape to protect the side of the AS. I carefully peeled the vinyl strip back, and drilled out a couple pop rivets to free up the extrusion and remove the end caps so I could cut it. A few pop rivets and some sealant on the top edge, and back it went like original.
Wife absolutely loves the fact that she can take a shower as hot as she wants, and as long as she wants, as long as we have water and propane...so from my point of view, it's priceless!
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.