I live in a single wide trailor on a farm. There are a lot of stray cats around and they are living under my trailor. Recently they have started spraying, due to being in heat. When I turn on my heat the smell BLASTS though my heating vents. Can I drop something down the vents like moth balls or another smelling agent.

You are ot of luck until you get rid of the cats under the trailer or enclose the trailer lower sides and back to prevent them from getting under there to start with. A trap or good pellet gun has worked in the past for me.

Get a dog that hates cats :), put some skirting on that trailer that keeps out all pests, then see if the odor dissipates after some time. If it does not you may have a leak in your ductwork and/or you will need it cleaned.

I`ve heard leadpoison works pretty well

I`ve heard leadpoison works pretty wellWhat caliber of "lead poison" do you recommend?:D

At least a 22 caliber is recommended around here in WV,, from a long distance puts a little sport into it

Though a fan of "lead poisoning" myself for vermin larger than 1/2lb, this can have various outcomes. Lets deal with the most immediate problem first.

The cat's urine is pretty powerful stuff. If it is the male's "marking" spray, then you need to find exactly what it is that is being sprayed and replace it, if possible. If you are smelling it in your ducts, I am guessing you may have some duct damage under your trailer, whether it was caused by the cats or not. Inspect the ducts to ensure they are still intact.

Closing in the bottom of the trailer is a good thing for many reasons, from keeping pests out, to aiding in temperature control.

What part of the world are you in? If you are in an area prone to mice/rats, snakes, other smaller vermin, I would suggest not completely getting rid of the cats, as they are what would be keeping the other vermin at bay.

If you choose to thin the 'herd' as opposed to complete eradication, then selectively "lead poison" the males, as they are the main culprit for the wonderful smell. Remember to "poison" them out in the open, no while they are under the trailer, because you have to be able to retrieve them after the act to dispose of them. You don't want dead things under the trailer, you'd rather smell the spray!