There is no need for a primer you will apply a flash coating then a final coat.
If you have an existing epdm roof, it is important for you apply a very light coat (a flash coat, approximately 4 mil or the thickness of 2 pieces of paper). Failure to do so will result in swelling of the membrane which will require additional days for the product to settle. The final coat will be at a 16 mil thickness. Divide the length of your rv by the number of remaining gallons, this number will tell you the length of the sections needed for each gallon. For example, if your trailer is 24 feet long, you will need 4 gallons. One gallon will be for your flash coat. The remaining three gallons will be for the final coat. After your flash coat, divide your 24 ft long tailer by 3 (the number of remaining gallons) resulting in eight-foot-long sections. Each of your remaining gallons will cover one of these 8 ft section.