Thanks for the info.
In my trailer, this circuit is the purple wire, and I have had the same problem. I found that the fuse panel was heating up to the point of damaging the purple wire that connected to it. There doesn't appear to be a short, but the fuse does keep blowing, as you are experiencing.
I have the original Univolt converter, and I strongly suspect that the metal fuse holders in the fusebox on it have deteriorated and since they will not keep enough tension on the fuse to maintain a good connection, the loose connection increases the resistance, it heats up the fuse, which then blows.
This is only conjecture on my part so far. I purchased a new converter and fusebox late last year and I mean to install them when I can get back to the trailer in May.
I also read on here that not having a battery installed in the circuit can cause problems with the Univolt's operation, which is why I asked you if you have one.
Good luck with your problem, and please post any information if you manage to repair the fault.
If it's to be, it's up to me.