If you want to create lookups against specific data elements or filter against them, then custom taxonomies will give you greatest flexibility.
There are methods to query against one or more taxonomies and terms. A starting point is here: https://codex.wordpress.org/Making_Advanced_Taxonomy_Queries
For the values that are unique to each RV, and therefore not suitable for a taxonomy, you might be better with a metabox of custom fields. A huge time-saver here is the Titan Framework plugin that makes creating and retrieving values a breeze. But you are still going to have to get your hands dirty to bring all the required data together for the XML output.
