I was in this exact situation, my cat is 10 years old and she was terrified of the outdoors, and would howl constantly for the duration of any car trip.

She howled a lot the first day, and it was heartbreaking. She stop crying when the truck stopped but would start again whenever the truck was in motion.

She howled for the first few hours the second day the truck was moving, but would stop when she could hear my voice. I spent most of the day talking to her, or just on the phone.

She tapered off the crying very quickly over the next few days, and by the end of the first week, she would only howl a little bit when the truck started moving.

Halfway through the second week she wouldn't cry at all, but she kept herself tucked tight into the back corner of the sleeper.

We went home for a couple days, and the first day back out on the truck was exactly like the very first day, but the second day had only a little crying when the truck first started moving and none after that.

End of the third week she actually came out of the sleeper while the truck was moving and rubbed against my ankle.

It's been a few months and she's perfectly at home in the truck now.

I didn't think it would work out, but it has and I'm extremely happy about it.

There's hope.