If you trace the battery leads (on a standard factory SMB build) from the house battery, it should go to either a standoff lug or the separator. Sometimes there is an inline fuse but not always depending on the year built.
The problem is you're saying the battery is dead. Normally you could attach a charger at the lug or separator on the house side. But with a dead battery it would be best to isolate from the van's 12v system to try to bring it back to life. A high amp charge on some types of batteries like wet cells can produce heat if the battery is dead. Bringing a battery back to life depends on the type battery, its age, how long it sat dead and a few other factors. If you plan to do a high amp charge, I'd suggest pulling the battery. As you mention, some of those little battery tenders have a rejuvenate feature but in most cases they don't do much if the battery has already gone south. It might charge but deplete much faster than a good battery. With an AGM, equalizing might be possible but that generally applies to weak batteries, not those discharged for long periods of time. By what you said about the battery, it does sound like an AGM but I'm just guessing.
Lastly you should look at the connections. Anytime I have battery problems I always check the connections before jumping to conclusions. After that you can pull the battery, bench charge it at a fairly high amp setting, and then do a load test. I had my van at a Ford dealership and I left the fridge on by mistake. It took my 2 Lifeline 4D batteries down to 9 volts. My inverter charger was able to bring them back thankfully. Lifeline said to charge them with at least 15 amps for a couple of days.