Keep in mind those rates you quote are teaser rates based on having excellent credit 750fico plus  and great income. The 20 year number is unwise as a rv will be underwater for almost the entire 20 years .. if it lasts that long and will be junk value at the end. 
Also 15- 20 year numbers aren’t available on older units .. like 7 or more years old and certainly not at the interest rates you are looking at 
I second the get some “real” rate quotes based on your credit situation and purchase choice.  I think you’ll find a modular or home rate will be competitive