Distance from the RV park's main electrical service is most likely the primary cause of voltage drop. The farther the electricity has to travel, the more voltage drop there will be. Heat would be a secondary cause. Conductive materials such as copper and aluminum (the two most common metals used in electrical installations) lose efficiency in higher temperatures. In either case, the solution is the same; throw more copper at the problem. Copper is a better conductor than aluminum but is more expensive. Depending on the distance from the main electrical service the conductor size for a 30 amp circuit should increase from the standard 10 gauge wire to 8, 6, 4 or in some cases, even 2 gauge wire.
Unfortunately this is where reality intrudes. Whether it's a new or old RV park, upgrading the electrical infrastructure with larger gauge conductors to minimize voltage drop is a massively expensive project. Even with a decent profit margin, the owners of the RV park would have to think very hard before making this kind of investment in their infrastructure.