There are several issues to the "marine" vs "non marine" debate.
Yes, engine manufacturer's jack up marine prices and non marinization OEM parts can often be purchased from tractor supply shops much cheaper. Even some marinization parts like oil coolers can be bought from US suppliers.
Some marine labeled equipment is exactly the same as its RV equivalent but much higher priced. There might be a little justification for this, because the warranty rate for marine stuff is probably higher than RV stuff. The marine environment is tougher.
And some marine equipment has subtle differences from its RV equivalent- stainless vs painted or galvanized housings, chrome plated brass vs pot metal, marine UL certification for electrical items, etc.
Some of the marine stuff is worth it, some isn't. It is often difficult to know which is which.