We live in Scotland, and have camped, had a caravan and a campervan, for about fifty years, and have always had our dogs with us (at one time we had five Scottie’s, altogether in a caravan - that was fun 😵💫). Have never had a problem with midges on the dogs, and we never used any sprays on them, apart from their usual flea and tick treatments. The midges were probably too busy enjoying my fair Sassenach skin 🤣 to want to try and get through a dog's thick coat. We are on dog number ten, and I cannot recall any of them being bitten by midges.
You could try those mosquito coils that you light. TBH we don't go to the west coast in high summer because of the midges, we go either up to May, or after September (the tree colours are wonderful). Plus it's quieter then.
Hope you have a lovely time in Mull, it's beautiful there, nicer than Skye, we think.
Lesley