try to get an antenna cable extension, and put the antenna near the trailer window.

Or maybe you might want to make your own antenna using a can.. search with google for 'cantenna'

you may also look at radiolabs.com for antenna booster/amplifier.

All of the above is applicable only if your wifi card has removable antenna.

To use a repeater or range expander, it might involve some configuration... look at linksys.com or google it.

Good Luck