One of our dogs gets very nervous and howls and pants

Try Desensitization training. This is best done under a professional dog trainer, but you can do it your self, if you are committed to completing the training.

It is commonly done with dogs that are afraid of thunderstorms, but will work well with any stimuli, when properly done,like riding in the MH. I have used it successfully with a dog deathly afraid of cars passing by when out for a walk.

Here is a link that explains it from the ASPCA: Desensitization training