I use whey isolate. It is a bit pricy as you've mentioned but the convenience is hard to beat.

If I had to go without it would be lots of beans, tofu, and seitan for me since I am vegetarian.

More or less yes you did American English correctly. More often I just refer to them as semis without adding the truck part, some also call them tractor trailers. It was clear what you meant though so only need to change it slightly if you are going for full American English