You can ask your insurance agent if they will write your policy as "agreed value coverage" which will give you a fixed amount IF the vehicle is deemed a total loss. If they will, they may require an inspection and if they agree, then you can use NADA values, or any other book that they allow. They will not allow you to pick a $100,000 value for an RV that is only worth $50,000 per the book, BUT, if you have some very special modifications, most companies that write agreed value coverage will allow you to add some of their value.
Do not expect this type of coverage to be the least expensive.
Actual cash value is usually determined by an average of various value books that are out there. Any improvements and maintenance items are usually not considered in the final value.