The yellow jackets are wasps and both species you have are part of the pest control system Ma Nature uses. I don't use and don't advocate the use of carbaryl often but this is one case where its use is necessary.
Make a small shelf to put near the yellow jackets entrance and put some of the carbaryl on the shelf so the yellow jackets have to land in the carbaryl to make entry. They will track this stuff into the nest and eventually kill the queen and larva in the nest.

Lesli, I'm sure what you are calling "yellow jackets" may be those Polistes or guinea wasps. Are they yellow/brown with maybe some mahogany spots? True YJ's are ground nesters. The ywellow wasps are the more agressive of the two. I have the black/red ones here, too. Much more laid-back than Polistes.
Two different species will fight--just like ants. My suggestion is to take out the yellow pnes. Use a good spray bottle filled with ordinary soapy water. It will kill on contact. Soapy water won't harm any plants or animals. I've actually shot wasps right out of the air with squirts of soap water. It doesn't take more than maybe 1 tsp to a quart of water to be effective.
Hope this helps.

I meant to make it clear to NOT harm the red/black wasps. That's a good species.

You need to treat the exterior of your trailer this spring before they start building new nests, Although I am in the pest control business I too am against the unnecessary use of pesticides. But a wasp infestation can be a potentially dangerous situation. There is a new product that I have found made from plant oils Called EcoExempt IC. It works well. (http://www.ecopco.com/products.htm)

If the wasps are nesting under the siding, are there openings in the siding the should not be there? I would think your first course of action should be to make sure your siding is properly nailed/screwed/etc. on and any cracks, holes, or gaps filled in. Make your home less attractive to the critters. If you still have the problem, then you can start using the solutions offered above.

Peace,

- Sequoia