So-called repositioning cruises typically sail during the fall and spring, when cruise lines move their ships to better climates (from the Mediterranean to the Caribbean in the fall, for example). These cruises often last longer than standard ones, yet cost up to half as much per day per person. Cruisecompete.com can point you to travel agents who book the trips.
When the seasons change, rental companies entice travelers to take their RVs to other parts of the U.S. to meet expected demand. This past June, for example, El Monte RV touted an 11-night one-way rental of a 23-foot-long vehicle from Atlanta to Boston for $539—a 70 percent savings (elmonterv.com). Along with El Monte RV, Cruise America is a big player (cruiseamerica.com).
In the spring, rental agencies shift part of their fleets from Florida to other states, following the tourist migration. One-way rates often start at $10 per day for unlimited mileage on midsize cars—with no drop-off fees. But you won't find these deals by using the standard booking engines; you'll need to look on the car-rental companies' websites (either on the home page or on the "special offers" page). You may need a promo code to claim the discount.