Ahead of time: Steam chopped potatoes in 1/2 cup water for 10 minutes until just fork tender. Drain excess water.
Preheat grill or campfire to medium heat or flame.
In a mixing bowl, combine mayonnaise, vinegar, sugar and mustard. Season to taste with salt and pepper. Add cooked potatoes, bacon, celery, and onion and stir until evenly coated.
Layer two pieces of heavy duty aluminum foil on top of each other. Pour potato mixture onto foil. Fold and seal edges together to form a packet. Grill or cook over campfire for 35-40 minutes, rotating every 10 minutes.
During the last 10 minutes of cooking the potato salad, add sausages to grill grate and warm according to package instructions. (Alternately, you can add sausages to individual foil packets with single-serve portions of potato salad and cook together, in step 4.)
Serve sausages atop Martin’s Hoagie Rolls with Dijon mustard; German Potato Salad on the side.
Recipe adapted from https://www.garnishwithlemon.com/grilled-german-potato-salad-sausage