Unlike a home, you likely will not get a blanket "pre-approval" from a financing institution of an RV. A pre-approval for a home is protected by the fact that the pre-approval is predicated on an appraisal, on verification of income and numerous other items. And even with a pre-approval for a home, the home would need to meet certain criteria such as being stick built, not a modular or mobile home, on owned, not leased ground etc.
With an RV loan, the value of the collateral, the condition of the unit, the type of unit, the make and model as well as the model year would all be important to a lender before they would actually fund a loan.
Some lending institutions MIGHT give you an idea of what they might do, but the probably won't actually give an approval or denial without you submitting an actual deal to them, since that might run afoul of many of the lending laws. For example, they couldn't say NO without a formal application that considered the actual collateral and down payment. They also couldn't give a blanket YES for the same reasons.