Blown a fuse?

Probably a fuse how much power does the inverter draw? More than a 5amp fuse I wager. Better using a gas kettle though

Would it be a fuse on the battery? Where would I find it? 

don't plug an inverter into a 12v socket ( what power was the inverter drawing when plugged in ). A low wattage kettle say 1000w will draw just over 4 amp where a USB socket will be well under that so an inverter will possibly draw more if the inverter is bigger than that of the kettle. (if that makes sense ) 

 When you say the usb plug is connected to the battery, I take it you mean that your motorhome has a ciggy lighter socket & you've plugged your inverter into this. If so the motorhome will have a fuse box somewhere (do you have a handbook?) and a fuse will have blown in it. Your handbook will have a key/legend as to what fuse is what.

Never seen a fuse IN a battery, but if the battery is in a protective case (as it should be for safety) there might well be a fuse on or in the case. Could well be a wee round thing about the size of the end of a lipstick that unscrews.

Do you have/can borrow anything else that is 12 volt powered and has plug to fit the socket on the battery?  Test it works somewhere else (e.g. friend's car) and then try your under-seat battery socket.

I don't think you have done much damage.  "Much damage" is the wires going red hot, melting the insulation and producing lots of toxic fumes. That is why we have fuses.

My inverter is bigger than a car battery and is connected with Anderson connectors (look up on e-Bay). But it has to run the microwave, espresso machine, and heated rollers as well as the kettle.

The round 12v sockets are normally rated at 10A or 120W so if your inverter is greater than 120w then the socket is going to start getting hot. A 12v socket was designed for low power equipment. You have probably blown a fuse. If its Fiat based then have a look down by your right knee and you should see a panel with a screw in it. Fuses behind the cover. Have a look in the vehicle handbook for which fuses do what. If its not in there then you need the book from the converter which will tell you where they have hidden all the fuses.

Next job is to buy a kettle that works on gas. The old one with a whistle is always a good buy. Using valuable battery power to warm up a kettle is not the way forward.

The problem you will have if you are not very careful is something very expensive is going to go bang. MHs are not the cheapest of beasts to fix so please be very careful what you plug in and where. Hopefully all you have done is blown a fuse.

Enjoy your camping.

I'm just going to put the kettle talk to rest - it was the first thing that was to hand. I'm not saying I'm keeping it, it was sat in a cupboard when I got the van and thought it might be safe because someone had obviously been using it before me. I'm not a kettle queen, I'm not keeping it, I have a gas kettle and will happily even just boil stuff up in a saucepan if needs be...it was just the first thing with a plug that i saw to test the inverter,

I'm literally just trying to make sure my battery won't explode anytime soon 

Now, onto the matter at hand..

The 12v plus wasn't the one in the cab, it was definitely going straight to the battery.

It's definitely something to do with the socket/battery as all the 12v devices I've tried have worked fine in the 12v socket going to the solar battery. 

The socket itself looks like the one pictured, only it doesn't have a plug - instead the cable's innards are fixed directly to le battery. Is that...normal?

Now, let's not get snobbish - this is a 16 year old beauty we're talking about. And I'm all for a bit of MacGyvering where appropriate...

It's definitely something to do with the socket/battery as all the 12v devices I've tried have worked fine in the 12v socket going to the solar battery.

As the song goes - 'It ain't necessarily so'. The socket that you plugged into cold easily be on a seperate fuse to any other. You talk about one battery as a 'solar battery' and one as a 'leisure battery'. 

The socket itself looks like the one pictured, only it doesn't have a plug - instead the cable's innards are fixed directly to le battery. 

What without a fuse? A photo would speak volumes. 

It is very bad practice to wire anything direct to a battery without fusing it, you run the risk of starting a fire.  In the picture of the socket you posted the fuse would be in the plug and if this has been cut off to wire direct the the battery you have lost the fusing. How have you checked it isn't working? The inverter may have a fuse or it may have a trip switch. If you have been pluging some other device into the 12v socket  and it is not working then the socket itself might be damaged. I supose the battery is not flat? My advise is not to use the socket until it has been fused.

peedee

The socket in the picture is certainly an add on and would not have been fitted by the converter. I would have thought that the internal wiring has been fried if it were connected straight to the battery without a fuse. The very thin wiring would certainly struggle with a plug in inverter. There is a chance that they put a fuse inside the box just to be on the safe side but fuses and parts cost money. 

I would seriously consider removing the unit especially as it does not have a fuse in the lead. Don't forget that these units are only designed for light loads and a maximum of 120w.

If you need 12v sockets then they should be fitted correctly to the MH wiring system so that they are switched off when you turn the power off. This will save you running a battery flat by leaving something plugged in. Just remember that the 12v sockets are for light loads under 120w. It is possible to get a heavier duty socket for 12v they are Hella Plug and Sockets and have a rating of 16A or 192w.