Most new Rvs have a similar percentage mark-up and can be discounted accordingly. The main difference is that higher dollar rigs (larger motorhomes and high end brands) have more absolute dollars in the deal, so a smaller percentage is needed to cover the dealers fixed costs. Net is that you can get a slightly better discount on higher priced rigs.
Car & truck dealers operate in a more competitive environment and their MSRP leaves them with a lower (but not tiny) margin. Bottom line is that discounts are nowhere near as large as RVs. Te customized "RV hauler" trucks might be an exception, cause the MSRP cmes from the customizer, not from the truck manufactures (Ford, GM or Dodge). I would say that 15%-17% is the likely discount on big diesel trucks, but that will vary by region, model demand and time of year (Winter is a good time to buy). I haven't bought a big truck in ages, so may be out of touch on the current market. The deal can get complex because there are often sales incentives, rebates and such from the manufacturer that figure into the pricing.