When i have a tube that won't hold enough air to let me search for the hole (or holes, usually) I either fold, clamp, or tie the tube into segments. That way i can localize the hole---and often, I can work my way around the tube, folding over a section and inflating the rest, then folding over the next section, until I have narrowed down the hole's location pretty well. Then I fold that section, clamp it (with anything---two small pieces of wood and a vice grip?) and get the rest of the tube really puffed up. So far, every time, I have been able to slowly open up the clamped part and find the holes----and by every time, I mean, 'in more than fifty years of doing this" so I at least, have faith in the method.