Put seitan chunks into the large mixing bowl with marinade and briefly combine. Let it marinate while you continue cooking, chopping, and preparing the remaining ingredients.
Fill a pot that can hold a steamer basket insert with a few inches of water. Bring it to a boil. Put the potatoes and sweet potato slices into the insert and steam for about 15 minutes. Test the doneness of the potatoes with a fork. If fork tines can go in easily, the potatoes are ready. (They should be easily pierced, but you don't want the potatoes or sweet potatoes to fall apart when skewered.) If the potatoes aren’t quite soft enough, remove the sweet potatoes and continue steaming until done.
Add the bell pepper, onion, mushrooms, steamed sweet potato, and steamed potatoes to the mixing bowl with marinade and toss to combine. Marinate for 10 minutes. During the last couple minutes, add the sourdough chunks and toss those with the marinade as well. The bowl will be heaping full at this point.
Heat the outdoor grill to 500 degrees. Put all of the ingredients onto long metal skewers, alternating ingredients between each one. Ideally finish the end of each skewer with a sweet potato slice, so that it can be next to a marshmallow when all is said and done for maximum kitsch-effect. Don't put the marshmallows on yet, however.
Grill the Thanksgiving skewers for 10 minutes, rotating a couple of times to evenly brown all sides. When the skewers are done, set them aside.
Spear each marshmallow with a fork and toast them over a small flame. You can use your gas stove or a candle. Once each marshmallow has browned, carefully top each skewer with a vegan marshmallow. (Don't burn yourself on the fire or metal skewer.)
Serve right away with cranberry sauce, cranberry salsa, and/or gravy.