Financing

You don't need to buy the RV first but you should have narrowed it down to a few possibilites so that when you do buy the truck you'll know how much of a truck you'll need. If for example you have it down beween 3 RV's, take the weight numbers off the heaviest RV and you'll know, worst case, how much truck you'll need.
Also if you do get it narrowed down, you can come back here and post what you are looking at. One thing about this place, not many are shy about telling a newbie what they need.