I had a rescue hard keeper, tried all of the above and nothing helped (he was 21 at the time). Anyway, he got really sick on me and the vet prescribed antibiotics for him. Not only did he snap out of being sick, he started putting that weight on I'd been trying for. Matter of fact I actually had to throttle back his feed for the first time as he was getting chubby.
Called the vet to see what could of changed, he said some older horses that have been neglected will get a low grade infection that their body just can't fight off. If that was the case the antibiotics would of got him over it and allowed him to put on weight. He wasn't sure if that was really the case with him but it was one possibility.