As mentioned by Johnny Lightning, use the insulating foam sealant
EVERYWHERE under your TT. I went through 2 cans of the foam spray, (goggles, mask, gloves, log sleeve shirt) when I did the under-bell foam project. I made 2 trips around the bottom and foamed
EVERY opening that I could find, no matter how small it was. I also have the Winter package, so there is a hard plastic under-belly that rested on the frame on each side. I foamed the entire length on both side where the plastic went into the frame mounting. There were a number of small gaps that a little critter could have gotten into the TT. I also did the front and back cross members.
Don't stop there. This is where you need to think like a mouse... On the front cross member there was a 3/4" hole (could see into the under-belly, so I caulked it too. I looked under the TT and there is a spare tire hanging under the trailer, lowered it and found another 3/4" hole where the cable comes out of the under-belly to raise/lower the tire. I used a plastic 4"x4" blank electrical box cover. Drilled a hole in it slightly larger than the cable thickness and cut a slot from the outside end of the cover to the hole and screwed it into the under-belly with the wire in the hole. I then caulked the slot .
There were also small holes where the fridge components are (outside vent area) caulked them also.
I did this in 2012 when we picked up our TT and have been critter free since then. I felt it was better to stop them before they got in, because they can do a lot of damage. DW also does the dryer sheets and 100% peppermint oil defense plan. I also sprinkle the Sevin dust under the TT to keep the ants and other bugs away from the TT.
Wife is happy, then I am happy!!!!
Good Luck
Don
My Registry
RVing with SOLAR