Look at eBags TLS weekender. Comes in regular and a slightly shorter version. Pretty inexpensive and durable. Good way to experiment with a one bag approach.
They are often on deal and a very good value.

BTW, I like Thule (have their sling bag) and their backpack would be great for what you have but you mentioned couple day TATL MRs. For that (at least for me) I'd want to take more in terms of clothes than I think a regular backpack could do. You're in a TLS or RedOxx Skytrain area then.



http://www.ebags.com/product/ebags/mother-lode-tls-weekender-convertible/143101

http://www.ebags.com/product/ebags/mother-lode-tls-weekender-convertible-small/241465?productid=10209114&rlid=DETAIL