I was going nuts trying to figure out why my new(ish) to me 2006 RT kept losing voltage to the Victron I put in to charge the LiFePo batteries. Took an hour of crawling down around and under to find out one of the two previous owners blew the fuse that goes from the starting battery to the isolator. Then instead of replacing it, they wired in a circuit breaker of indeterminate amperage via jumpers and electrical tape
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Best guess is that it is 20 amps because when the Victron wanted the full 30 it can use, the breaker kept tripping. Off for a second then resetting.
I understand why a shade tree would do that because finding the right replacement fuse was a biatchico. The closest replacement I could find is a littlefuse 0498070.TXN from Digikey. I could find a bunch of 70 amp midi fuses cheap but they are about twice the width and a little long to fit in the holder. It will be a few days until it comes and I'll post pictures.
I guess when they were changing out the coach battery they must have shorted it out. No other way it would have blown.