I believe your problem may be in the fuse panel....
On my 84 Excella, a 50 amp fuse, one of four, kept overheating and blowing. Heating is a function of resistance in electrical matters. Resistance is caused by a number of factors, one being the solidness of the connection.
After investigation, I discovered that the mounting bracket of the 50 amp fuse holder to the fuse panel plate, that was the bar that powered the other 10 or so glass fuses, had become loose.
There was no way to adequately tighten this bracket to the bar. And since it was in fact this attachment to the lower bar that powered the entire bar for the adjacent fuses, there had to be a solid connection made from the bracket to an adjacent bracket to provide this power source.
Therefore, I had to rig a jumper wire from the supply wire for the 50 amp fuse to another bracket on the powerbar side of the fuse panel to eliminate the loose connection that was producing the heat and blowing the 50 amp fuse.
Once the jumper was rigged to separately power the bar for the rest of the fuses, the 50 amp fuse for the main battery circuit functioned properly because electricity follows the path of least resistance and a solid connection had now been provided to power all the remaider 12V
circuits that had almost no resistance, and therefore created no heat not directly related to the primary circuit.
That may not be an understandable explanation, but I would look at my entire fuse panel where the suspect 50 amp fuse is in particular, and see if the bracket the suspect 50 amp fuse sits in is tightly mounted to the plate or if it is loose.