Banamine will help bring her temperature down. If it is respiratory, which I think likely, I'd give her something like Resflor Gold which already has Banamine in it. Talk to a cattle Vet again, and describe those symptoms. Hopefully one will give you a prescription without having to come out to see the animal. They can go very quickly from slightly lethargic and off feed to deaths door. I wouldn't wait on treatment. A temperature of 105 can be dangerous. There are certainly better things for what you are describing than LA 300. LA-300 is used more for treating eye and foot infections. Your calf sounds like one I lost a few years ago. He didn't seem all that sick and I thought I'd just keep an eye on him. I might not have even noticed anything was wrong, but we were doing an inventory of calves and all of the other calves got up and gathered around near the ATV. We found him maybe 50 feet away hiding in the weeds in the shade. When I got him up he walked slowly to the cow and sucked half heartedly before lying back down. We were busy and didn't check cows the next day, and when we went to check two days later the cow was with the herd and he has no where to be seen. We found him not far from where he'd been before and he obviously had respiratory issues. We got him to the corrals, took his temperature and treated him with Nuflor (same as Resflor, different company) and Banamine. For some reason he went crazy in the chute. We let him out and he just laid down and by morning he was gone.
Last year we had another one. Just lethargic and not hanging with the herd. That time I didn't wait. Brought him in and his temperature was 105. We treated him with Nuflor and Banamine and his temperature came down to 101 and he was back acting normal in only a day, but by then we had more calves hanging their heads. I hadn't vaccinated yet, so we brought in the whole herd vaccinated and treated any with a temp over 104 with Nuflor and Banamine. I gave all of them Enforce III as advised by my Vet. He said the nasal vaccines have a much quicker response time. We got lucky and stopped what I think was about to be a wreck. We had recently moved the cattle up along a fenceline that bordered the neighbors cattle, so I expect they picked up something from them.