I have a staic caravan and we have a small patch of garden

A bonsai needs to remain in its very small pot and needs a lot of care and attention, difficult if you are not there all the time. If it is planted in a trough it will start to grow bigger. You don't say where your caravan is sited - this makes a difference to what will do well over the winter when you aren't there to look out for it. Perhaps a miniature azalea might do well if you use acid soil, but you would need to make sure it was kept moist through any dry weather. How about a grass? Some look very oriental and there is a huge range of colours and sizes to choose from. Or there is a bamboo that doesn't get too big. I can't remember which it is but Bamboo on here knows - you could send her a private message if she doesn't see this. (The avatar name is a coincidence)

The dwarf Acers look oriental. (Acer palmatum disectum atropupureum is a good one to look up) but they can be tricky, don't like the cold or wind or too much sun but are lovely

I came across this very useful website recently. It is interesting in that one of the first things it says is that an apprentice bonsai grower is given secaturs on his first day but not allowed to touch a watering can for four years. Watering is so important. http://www.helpfulgardener.com/bonsai/03/bonsai.html

I like the ideas of the miniature azalea or rhododendron. I have two, one in each foot square pot and they do very well here in Scotland. I have small bulbs in the pot. I don't need to water them. Plastic pots retain water better than pottery versions.
Your trough will need to be lifted up on bricks to allow it to drain and to stop the soil from going sour. There are lots of grassy type plants that would look good, saxifrages and sedums would also be good candidates.
Click on J and the Japanese gardens in the A-Z below. Also P and patio ides. It is such a personal choice maybe you can get some inspiration to make your own garden just as you want it. Do you live there permanenetly or is it a holiday home where maintenance would maybe be a problem?

the only real way of having bonzei is to plant them in there little pots within the trough as they need atention every year and when older every 2 years plus as said in summer they need watering at least every day . a lot of people think they have to come in the warm but if there trees that naturaly live outdoors in our country thats far from the truth .