Do you get a good enough seal on the buckets to not lose water during transport? How do you plan to get the water transferred to your RV? Which containers would make that easier and also allow you to run and get more water if necessary? If you are going to use a water pump to get the water into your RV, it would probably be less hassle with the larger tank. If you are manually transferring, it probably doesn't make much difference since you would be filling a smaller container from the large one anyway to pour it in the RV.
Would you ever be in this position of needing added water in the future and which method would be easier for you to store for future use? The large tank would definitely take up more room than 13 nested buckets.