recommendation for mattress pad/cover for the stock mattress

We have the Costco topper too, and honestly the mattress still sucks. My suggestion is buy a new mattress. I am pretty sure the RV mattresses were designed by the same guy that built medieval torture chambers. Pads "help" but they can not mask the shortcomings of a bad mattress from the start. Get the measurements of your mattress, as RV sizes are not normal. And visit stores like the Denver Mattress store and others to see if you can find what fits you best.