I really like the Marmot sleeping bags. The Arroyo (30F) and Pinnacle (15F) are the slightly heavier and bulkier versions of the Hydrogen and Helium. The difference is in the quality of the down - Hydrogen and Helium use 850 fill, Arroyo and Pinnacle have 800 fill. So the higher the rating, the more loft it has, and so the less they have to use, which makes the bag lighter. Also there are differences in the materials used: The Arroyo has slightly heavier liner, and lighter weight outer material, whereas the Hydrogen is the other way around. You can see the specs on their site:

http://marmot.com/products/arroyo
http://marmot.com/products/hydrogen
http://marmot.com/products/pinnacle
http://marmot.com/products/helium

I have the Pinnacle currently, though I kind of wish I got the Helium instead since it's a little lighter and not quite as warm (I think the Pinnacle is probably too warm for most activities which I use it for). I think the Hydrogen would be almost the perfect bag for a trans-america tour - only 1 lb 8 oz, and very compact, and full length zipper (so you can open it out almost completely to use as a quilt on hot nights). The Arroyo is nice too (and cheaper), and it weighs 2 lbs. I had an Arroyo on my 1998 TransAm, and it worked fine - though it seemed to not have very much down in some spots, in some places it felt like it was just two layers of material there with no down at all. After I got back from the trip, I didn't store it very well and after a while it didn't seem to have much loft left, so I gave it away. But I have since learned about how Marmot will often re-loft these bags for free or nominal fee (if they've been otherwise taken care of), so now I wish I'd kept it. My current Pinnacle is very nice, but rather warm for summer use.

If I had the choice now, I'd probably get a Hydrogen for summer tours in the USA, and a Helium for more iffy conditions where it might get cold. In fact, I think the Hydrogen could probably handle most conditions, especially if you use a layering approach - take along something like the Sea To Summit Reactor Extreme bag liner, or a nice silk one, and that'll add some warmth to the bag on cool nights. Then add wool underwear, wool socks, and a wool cap for really cold nights, and I think you'll be able to handle a very wide range of temperatures. On hot nights, just use the liner (and have the bag there next to you ready to pull over as a quilt, since I find I often cool down in the night and get cold around 1-3am). This will see you for most conditions you'll see in summer in the USA, I think, especially if you have a decent tent.

It's true that you should keep down bags dry. However I have never actually had my bag get wet. If you have a good tent, and take care not to camp in locations that are going to get flooded if it rains in the night, and carry the bag in a waterproof stuff sack, then I don't think it's going to get wet very often, if at all. In any case, you're not going deep into the wilderness here, it's a road tour in the USA. If you get stuff wet then you can just take a day off in a cheap motel somewhere to dry off.

Neil