You are looking for a booster for Verizon's signal, right? Not a wifi booster? If this something you know you need, i.e. you will be in a place with little or no cell service, or just something to help out if needed? And I assume you need a booster that works with 4G LTE and not just 3G? Will it be just the Ipad that needs it? No cell phones in use?
The best cell boosters come from weBoost, formerly Wilson AMplifiers. The good ones are pricey and the less expensive ones don't help much. To be effective, you need a substantial outside antenna, usually at least several feet above the RV roofline. That connects to an inside antenna that re-broadcasts the signal. If the Ipad can be very close to the inside antenna (like 4-10 inches), you don't need much for an inside antenna, but if you want to walk around the inside of the RV and talk on a cell phone, you need a top quality inside antenna as well. A "whole house" or "small office" size, not the typical car-interior antenna.
I used the weBoost "Home 4G" model this summer in the4 North Carolina mountains and it helped (went from zero service to moderate). Antenna mounted on a mast at the rear of our coach and 18 ft above the ground.