Hi, i came across something quite by accident, or rather my kids did, they were messing around with the airbed in the kiddies pool, and bubbles were coming out of where there was a hole.
So i guess, you could try this, if you have a paddling pool, or even better maybe pumping the bed up whilst it is in the water, and you should be able to see where the air escapes.
If you are unable to use the water method: use a tiny, light feather: pass the feather slowly over the whole bed until the puncture is found.
If you still can't find the puncture, it is more likely to be the valve. Paint some fairy liquid over the valve and see if air is escaping. It will bubble if it is faulty.
I had the same problem...a very slow puncture. My sons friends would go to sleep on a firm mattress and wake up on the floor! Due to the size of these airbeds, it is difficult to immerse in water. I rubbed the sides with washing up liquid and a small amount of water and just watched, eventually a tiny soap bubble appeared, the hole was the size of a pin hole. Out with the bicycle repair kit and it now works perfectly. Hope this helps.