Why does my r.v. Die after about 5 minutes of running?

What is your question?

I’ve replaced the batteries the starter the relays and the alternator and it still dies after about 5 minutes of running and loses all voltage and power. I can jump start it back up and voltage will be fine for another 5 or 10 minutes then it drops off and dies again.

How long has your 1990 Ford E-350 Econoline had this problem?

Just started