https://www.rvusa.com/rv-guide/2010-...l-specs-tr9523
Above is a link to the 2010 278rks. RVUSA has usually been pretty accurate in my use of it for specs. This one also shows a 38gal. tank. George indicated his 2002 has a 30gal tank which would indicate they changed tank sizes during that 6-8 year time frame which would not be surprising (if the link is accurate).
I've never worried about the gals. in any waste tank, I measure the capacity in days. Black tank is the same - how long before it "burps" at me. Also tells me if I need to start going #2 in the woods or use a bathhouse restroom...but gallons never cross my mind.
Edit: To clarify - I have thought about gallons in the holding tanks before purchase because I know the more gallons the longer I can go without dumping...pure laziness. After that it is what it is.