@Ray - I've traveled for almost 2 weeks with an under seat bag. As we know, if you can travel for one week you can travel for many. And yes, I've done it with winter clothes. I usually bring 4 shirts, 2 pants, a sweater, a skirt, 2 pairs of shoes. When I wash my clothes I bring them into the shower with me. Ive found that easier than sink washing, though I'll do that with undies for sponge bath and non-shower days.
If you bring a DLSR and associated lenses then you'll have a hard time. I'd say it's not impossible but very difficult. Your camera is a choice just like others desires to have more clothes, a full size laptop, etc.
A 30 liter bag is at the upper end of what will fit under most airline seats. This differs by airline. For example, Norwegian Air personal item is defined around 16 L. Southwest on the other hand has a huge allowance. YMMV.
BTW I did pack a 45 L carry on when I went for 5-1/2 weeks to South America. I was taking mountaineering gear (full hiking boots, full rain gear, 2 sets of thermals), jungle birding gear (binoculars), and gear for a cruise. Getting around LANs 7 kg limit was tricky (I put my clothes in my day pack and created a personal item). It all fit in my MEI Voyageur but I packed too much.
Edit: Lately I've need traveling with a "22 L" bag in winter (16 L by my measurements). I suspect that you could get your DLSR gear into a 25 L bag.