Fellow road warrior here (sales). Here's my actual cheat sheet:
McDonald's: Double Quarter Pounder with Cheese, no bun, no ketchup. Ask for it in a bowl or lettuce wrap. Add extra patties if hungry. ~3g carbs. The McDouble without bun works too for a cheaper option.
Wendy's: Dave's Single no bun, lettuce wrap, add cheese and bacon. Or the Baconator no bun. Their salads with grilled chicken and ranch are also solid.
Chipotle: Burrito bowl with double meat, cheese, sour cream, guacamole, lettuce, salsa (small amount). Skip beans and rice. One of the best fast food keto meals — filling and feels like real food.
Subway: Any protein bowl — they'll put your sub fillings in a bowl without bread. Turkey, tuna, or steak work well. Add all the vegetables, oil and vinegar dressing, skip the sauces.
Chick-fil-A: Grilled nuggets (not breaded!) with no sauce or ranch on the side. The Cobb salad no croutons. Their grilled chicken sandwich in a lettuce wrap works too.