I just went through this about a month and a half ago. The original Atwood detector shown in your pic has been discontinued for a good while now. Atwood was bought out by Dometic and they where offering the upgraded version, just under their name.
Finding an exact drop in and not having to deal with the mounting hole, is hard at this current time.
This one. Atwood/Dometic LP gas detector #36720 https://www.dometic.com/en-us/outdoo...detector-19503
While that is not an "exact" fit to the old hole, they used to offer an adapter ring that would make it fit. The adapter ring does not seem to be offered any more. In fact, there seems to be a shortage on the 36720 as they are very scarce and not in stock in my normal go to places. This one was the closest to fitting, some of our members who changed them out a few years ago, used the old original adapter ring and mounted the new on inside that.
Dometic does now offer a combo LP/carbon monoxide alarm. The 36681
I can find the 36681 tonight on Ebay https://www.ebay.com/itm/26515212193...YAAOSwXgJglXmm
I got stuck on Ebay in the past buying these, the dates on them where 5 years old when I got them. I'm not saying that seller has old ones, but check before you order. The Dometic's list out as 84 months for life.
But I can't find it on other sites for sale. They too seem to be out of stock a lot.
Since I could not find any of the Atwood/Dometic, I switched brands to the Safe-T-Alert brand made by MTI Industries. These folks
In my case, I opted to get the combo LP and CO detector as it has a end of life feature that it would tell you when the sensor was going bad. And that the friend I was installing it for wanted the CO feature. MTI does offer just the LP detector, but I could not find the end of life sensor in the manuals of them like I could on the combo unit.
I picked this one, https://www.amazon.com/MTI-INDUSTRIE...F%2CB092R9BR7T
The fit in the cabinet is very different. I had a panel of Sunline maple that matched the camper I was working on, and I was able to covert to fitting it in and looking really well. I was helping a friend with their camper and they have the detector in the dinette seat. See here, with the maple panel change you can't tell there was a big hole there.
The Safe-T-Alert brand is a lot more available at many RV parts selling places. They offer them in different sizes/shapes in both the combo units and the dedicated LP units.
Here are some of the places that sell them that I have had positive buying experiences from besides Amazon
I'm not sure you are going a direct drop in fit. And I'm not sure what the problem is with the Atwood/Dometic supply issues on theirs. Check the size of the model/brand you get and see how you can adapt it. Many do offer surface mounted detectors verses flush mount, that may create some options for you.
It is good you are looking into this. Any Sunline that still has the original LP detector in it, is well over due to be changed. The sensors go bad after a number of years. If you press the test button and it does nothing, the detector is no longer working. And there is no end of life alarm on them.
Please let us know how you made out on this. And how you where able to mount it.
Hope this helps