About the only reason to take out a loan when you really don't need one is to keep your credit active. Yes, not using your credit or having no recent credit history is as bad or worse than having bad credit.
Buying new for cash is really no different than buying used for cash. You pay and leave with the paperwork and the vehicle. The dealer sends the signed title to the state and you get the new title in the mail a few weeks later. As long as you are dealing with an established dealer with a large inventory, you should have nothing to worry about.
Look around the lot and add up the dollar amount for the new inventory sitting on the property. If there are 20 5th-wheels sitting there he is likely sitting on a million dollars in debt just for those. Your $40,000 worth of business is important to him. He has a payment to make also.