Most will protect the truck from serious water ingress. There are a few on the market that should help with out spending a fortune. Other than that ...a plastic sheet from B&q would work until you have sorted the issue out im sure
Ebay protec covers or Maypole or Kampa covers are the best. Dependant on budget of course..
As Kev and Carolyn say above - Maypole. Their motorhome covers are also sold on Amazon and they have a few listings on Amazon for different sizes. The link below takes you to one of the smaller sizes with others available and reviews. I've never owned one but considering getting one in the future.
If you have anything on the roof, sat dish, solar panel, TV aerial etc, they will need covering with old carpet or duvets to prevent corners from rubbing through the cover.
You can never secure the cover tight enough for strong winds.