This "might" help.
Does the inside of your furnace look like this? This is the model 7920 The 20 version of the 79 series.
Atwood 36543 7920 II 18K BTU Hydro Flame Furnace with B Vent | eBay
If it is, this "might" be the intake vent for it.
7920 II Atwood Furnace RV camper motorhome Trailer Outside Vent 79 11b Tube | eBay
I have never seen one of these in person, but know others have the 79 series furnace and still in use in the older campers.
If you can find the serial number, Atwood customer service may be able to confirm the part number of the air intake/exhaust vent. You can try here and ask for customer service.
Contact Us at Atwood Mobile
Sunline did not make this part, they just install them in the camper.
A local RV dealer may be able to order that part for you or have one. Atwood is one of the many popular brands in furnaces, hot water heaters and stoves.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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