I googled "Groceries Alturas" to find the name of the market I always see, as a Northwesterner myself. It's directly on your NW route (you don't even have to leave the highway; it's across the street from the famous Alturas Chevron where most Northwesterners stop to top off their gas). Try calling:
Four Corners Market
1077 N Main St, Alturas, CA 96101
I would call them once now, and once in August, to determine whether they stock dry ice, and a lot of it, and what their hours are. (Alturas is a small town, and while they make a good chunk of change when Burners pass through, a lot of businesses there shut down at 9 or 10pm.) The same Google search brought up Holiday Quality Foods and Rubios Market; you could google those too if necessary. Good to have a backup.
I always tote my trash all the way home, so I can't give personal experience with disposal--and you're right, the main site does not list trash sites for Northwesterners (on the route I take). Bringing drawstring bags that are allowed to air slightly at the top (when you're in camp) helps dry out trash, as does drying fruit rinds in bags made of netting before putting rinds fully away. Have extra bags so you can double up if something is leaky.
On the return, the first showering opportunity could be Cedarville (if there is a motel to be had--which there may not be; they're a tiny town and tend to shut down at night except for the friendly bar & grill that serves BBQ). I wouldn't count on getting a shower there, but it's nice if you see a sign or an open motel. Next opportunity would be an Alturas motel, after that might be Eagle's Nest RV park in Tionesta, CA, whose proprieter (name of Wedeliver
) is an ePlayan. There are showers.http://www.eaglesnestrvpark.com/
The earliest I've ever stopped for a shower was actually Chemult, OR, which is a good halfway point for sleeping on the journey back home.