FWIW: I pull my 16' medium-heavy Scamp
with a 2005 Forester, which has a 2.5L normally-aspirated engine and a manual transmission, and if I'm driving 65 or so (like I usually do on the interstate), I can leave in 5th almost all the time, unless I'm climbing a steepish interstate-type hill (which is to say, a minor one), where I downshift and/or let the speed drop as needed.
If I'm on a non-interstate with, say, a 55mph limit, I find myself downshifting to 4th quite a bit more, because not only can the hills be steeper, but at 55mph I'm not as far into the power curve as I am at 65. I think even if interstate and non-interstate hills were comparably steep, the power curve thing would in my case still dictate more downshifting than the interstate does.
Now the disclaimer - I have no idea whether the gear ratio on an automatic OD is comparable to that of a manual 5th gear, so all this may not apply so much to an automatic, but I hope it's at least some useful perspective.
(Just in case anybody's interested, I get about 21mpg at 65mph on slightly-hilly Minnesota interstates with no significant head or tailwinds. Is this typical for a TV in this class?)