The Ram Van comes from the factory with fuse boxes/panels located 1.) next to the battery or very near it in the engine bay. 2.) The left side of the front dash.
The third access panel you have is from the conversion company.
If none of the fuses are blown, you tested each with a hand volt meter and still have no power, then one of the follow is an issue, ... 1.) A fuseable link has taken the overvolted energy and burned up, 2.) A wire in the harness has fried. 3.) an electrical su-component that relates to managing power to those devices has shorted.
The fuseable links are down behind the battery on most of our Ram Vans. If you do a search in this threaded forum, there are diagrams and repair tips others have shared over the past months and years.
Did you use a volt meter ahead of time before installing the new DVD player????