Inverter DC supply fuse blows

[Times Two]

I have a Magnum 2kw pure sine wave inverter about 7 years old. Came down to the boat a week ago and found:

Remote MR ME indicates
- Red fault light solid
- Dead Battery text

Shore power indicator light on at plug
SP light out at panel and no power to panel.

Only loads running before the event was two dehumidifiers running on SP. No DC loads and inverter turned off.

Victron indicates all normal with the batteries and solar panels keeping the house bank on float.

Troubleshooting found DC 300A class T fuse between the battery and inverter isolation switch was blown.

Found no other problems so put it down to recent storms and replaced the fuse.

Everything looked good, all indications normal, shore power light at panel on. Ran the Air Con, dehumidifiers and all was well. After four days with no problems went for a sail. Upon returning and plugging back to SP, I found I had the same problem and the 300A fuse is once again blown. All other symptoms as before.

So guess I need to do more digging. Any ideas?

Thanks