I can’t add much more to the fine suggestions already given but I can offer some encouragement. My dog would stress out really bad in the car and trips were so miserable that I finally got some sedation from the vet. I hated to use it because it pretty much knocked her out for 8 hours and it took her a while to wake up fully, but it had to be done at the time. She was really bad in the crate so I usually didn't use it. Instead I tied her to the seat she could touch me but not interfere with my driving. It was not as safe for her as it would have been in a crate, but it had to be done.
But as time went on, she got better. Especially on interstates where the motion and noise was steady. One reason I bought a Sienna was to be able to use the crate on the floor, and for some reason she did not object to it and now rides pretty well. She still gets overly excited when I slow down (thinking “we’re HERE! I guess) but all in all she is a pretty good traveler now, and the sedation has not been used in a few years.
After I got a new HVAC system in the house, she had similar panic episodes when the fan kicked on. I am guessing because it sounded like rain, and rain means thunderstorm in her world. T-storms are still the worst for her. However, as time went on (only a month this time) she adjusted to the fan noise and not does not even flinch when it comes on.
So hang in there! I would try some of the remedies suggested here and also get some sedation from the vet to be used only if needed. Not taking the trailer is not much of a long term solution either
As for the towing, I think you might be pushing things with the Sienna, even if you are well under 3500 lbs. I have a 2015 Sienna and a 16 foot Scamp
on order, so hope the lighter Scamp
is OK with it, but I know the Sienna is far from a great TV. Perhaps some sort of towing expert in your area can inspect your setup? Also, I wonder if you know someone with a beefier TV who could drag your trailer while you sit in the car with the pooch, and see if that helps.