I am sure that many use a different approach but here is the way I have done when buying the five RVs we have owned.
First I determine how much I can afford to pay, or am willing to pay. This is my absolute top budget price and I will not go one dollar above it. (I have always paid cash and paid for it on delivery.)
Next my wife and I determine which RV (we always buy a 5th wheel) has the features, especially including the floor plan that suits our style of using it. We take our time looking as numerous different makes and models, with our requirments in mind. We select the floor plan based on our using style after considering the other features. The way we intend to use it is very important.
When we determine which one is our first choice, with the ask price not over 20 percent higher than our budget price, I sit doen and go into hard core negotiation for the new one. I either get it at my best price, below my top price figure, or I walk and go to another dealer or we offer on our second choice.
I have never paid above our preset budget price for each of the five RV's that we have owned. In fact I have always bought our RV at a somewhat lower figure than our preset top price.