Rule of thumb is to pull wire that is over the typical ampacity of the circuit being established. AWG 6 will remove the possibility of voltage drop, allow for upgrade to 50 amp service, allow for other devices to be used besides the RV. The cost between AWG 8 and AWG 6 is not that great.
I'd suggest to really consider the advantages of a 50 amp service, whether you have a 30 amp pedestal box or not. You can install a 50 amp or even 75 amp loadcenter at the storage building and pull wire from that to your 30 amp box and other receptacles in the storage building, lights, etc. When I pulled #6 into my garage from the house, I bought a 75 amp load center at Home Depot for around $25. Breakers were an additional expense, of course, but I now have a well distributed electrical system for power tools, a welder, and my RV.
Good luck with the power!