Hi smiley,
A bucket or two of nice hot water with a liberal dollop of car shampoo/wax and a bit of elbow grease will soon have your caravan looking shipshape again...
This will give you heart to tackle the inside...a good vacuum clean all over the inside and then a wipe over the hard surfaces with a soft cloth and a little cleaning stuff will soon have her looking good again...
Opening the windows and door and letting in a little sunshine (if you can find some) will work wonders as well!
When she's looking tidy and neat again ask a mobile caravan technician to carry out an inspection/service..... it will be money well spent.
All the best
paws