How about getting them tested for disease, getting them shots and spayed (or neutered) and putting them in your house. Yes, the yapping dogs will upset them at first, and they’ll probably hide out for a few days (to the point where you will wonder if they’re even in the house at all, they’ll hide so well) but then little by little you will see them more. If they’re feral, you might not ever get to touch them, but at least they’ll all be under your roof and safe and sound. If after a few months (or weeks if it goes well) you want to let them out you can, and by then, hopefully they will trust you and come back to the house.
I had a feral cat once – I had to trap her to take her to my new apartment, and once there, she never trusted me to touch her or sleep on my bed or anything, but at least I knew she had a nice, warm, safe home and she was taken care of. I had other regular cats, and I only put them together after getting her tested and spayed, and vaccinated.
After we lived in that apartment for about a year and a half, there was a fire, and I knew I would never be able to catch her to take her out, so I opened the windows in case she had to jump out (we lived on the 3rd floor). The apartment was not damaged by the fire, but the entire building was wet from the fire hoses, and when the firemen went into the apartments the next day to get any animals left behind, she saw them coming and jumped out the window, never to be seen again. She was about 10 at the time, and they said they saw her run into the woods. Oh well that was very sad but I consoled myself by thinking that at least I had fed her for all those years at my previous home, and then took her with me to this apartment and she had about a year and a half safe and sound (although a little skittish, she trusted me somewhat. Anybody else who came into the apartment never saw her, that’s how untrusting she was).
Sometimes local cat rescue organizations in your area might have programs through which you can spay/neuter and vaccinate for a very reduced price.
It seems like for you, it might be easier to integrate them into your home than have to deal with them being in the trailer and secluded. Plus, you won’t have to heat the trailer.