I have a 1985 toyota dolphin rv, equipped with the 22re. It has an air system that will discharge the battery, if it is not driven. The last time it discharged, I charged it and noticed the charge and brake lights came on. I brushed it off until I accidentally allowed it to discharge again. When I jumped it with my truck, it would run until I disconnect the jumper cables. Then it dies. A different battery solved the problem. I diagnosed the problem as a bad alternator. I cleaned up all of the contacts, then tested the battery voltage with the engine off. It was 10.2v ( It had been used via starting). When I tested it running, it was at 10.1v. This leads me to believe it is the alternator. I also tested the output from the alternator, via the wire and ground. It read around 3v. I have a rebuilt one in stock locally, but is there anything else I could check?
PS my 82 truck read at 12.1v when sitting and 14v when running