Nearly all RVs are lemons compared to even the worst modern car. Plain and simple.
Lemon Laws apply to the chassis in most states, but rarely the rest of the coach. And.... it may only apply to the first owner... so do your own due diligence for your state.
I think the chances of buying a used lemon diesel pusher is low, but beware of potential repair costs. Basically, the more it cost new, the more it cost to fix things. Also keep in mind that some owners with a problematic coach find is easier to take a loss trading it in, vs the hassle of applying a lemon law to an RV. The RV industry has very good lobbyists who've made sure the MFG always wins.
On the flip side... there are aftermarket "warranties" (really service contracts or breakdown insurance) you could purchase for a used coach.