blowing fuses in my Jayco Hawk O/B

Hi All, I ahve a small problem with blowing a fuse in my Jayco.
I have an American (brand name) converter and have wired it up with a 95a/h battery. My problem is when I have 240v running into the converter and then turn the switch on to charge battery it blows 1 of the 7.5a fuses thus cusing lights to go out.

If I disconnect the battery and do the above proceedure then run a multimeter acrooss the wires it is (+ and - feed to/from battery)it reads 13.15v, when I connect theswe wires back to the battery, it blows a fuse.
From the positive terminal I have a wire going into the single pole switch, from this switch iit feeds to the back of the heatsink. negative wire is feed straight to the earth block at the rear of the fuse box.

Help please.

Cheers
Nigel