Don't scare the poor guy! ;-) 12VDC systems are not that sensitive to typical loads found in haunted houses. Voltage drop on a line is a function of ANY ELECTRICAL SYSTEM (including 110/120AC and up). It is not a problem unique to 12 volt systems.
12 volt systems are "easier" to work with, but if you have to make custom harnesses, it takes a lot longer to make, mainly because you have to buy the connectors and wires and put it together from scratch. Also, for big modular attractions, you have only a limited amount of length to work with; for example, lets say 12 volt systems will only work across 50 feet of wire. you have to position the power source so that it's no more than 50 feet of cable-length away from the farthest haunt, like in the center of the attraction. if you have the farthest one 40 feet away, but have to run the cable up ten feet to go over an obstacle, then over 40 feet, then down 10 more feet, you're at 60 feet, which is 10 feet too many(in this instance. Actual lengths will vary depending on the type of wire, and some other things).
You are likely to only run into this problem if your application is a high-current application. (Don't try to run things with motors off of 12VDC without HUGE wire). Using 12VDC systems for things like LED based lighting and microcontrollers/valves won't cause issues with voltage drop for properly designed systems. We've been running 12VDC shows for nearly a decade, and have only had one issue on 12VDC line that ran THOUSANDS of feet... and even that was solved with a suitable capacitor at the microcontroller to suppy the current demands of firing the air solenoid valve.
Most folks with typically sized haunts will have zero problems with 12vdc systems across even multiple hundreds of feet using standard sized speaker wire cabling. If you do encounter problems, make sure you aren't running everything in your show from the same trunk off the power supply (you are running fused links, aren't ya?), and make sure your power suppy or battery system is up to snuff.