Stainless steel bench vises, usually fabricated from welded plate steel, are a thing, but the price is likely going to be higher than your budget.
A Yost 4” stainless vise, without a swivel base runs about $800 on Amazon.
Yost Vises SSV-0406S 4" Stainless Steel Bench Vise with Swivel Base: Amazon.com: Industrial & Scientific
www.amazon.com
Yost makes other sizes, and there are other options in stainless, but likely more expensive.
There are also bronze and bronze/aluminum vises made for work were non-sparking tools are needed, but those will likely run $2000 or more.
I think most people who need a truck mounted vise just use a decent but somewhat affordable steel or iron vise, then spray it down with wd-40 or Boeshield every once in a while to make sure it still functions.
Wilton also manufactures a “Hitch” vise which fits a 2” hitch, and which also comes with a mounting bracket.
The idea presumably being you keep the vise in a toolbox, and attach the vise when needed.
The Wilton comes in at least a couple sizes.