I also do a bit of surveillance and have a van setup for it. I have two battery systems besides the engine battery. Each are made up of two 6 volt golf cart deep cycle batteries hooked together. I have a marine switch to switch back and forth between systems or to have both online at once. They charge off a high output alternator or a built in 110v trickle charger I can plug a land line into.
These battery systems run all DC equipment like video camera(s), monitor(s), radios, scanner, fans, lights, etc.
Recently I switched to Optima yellow tops to avoid the lead acid batteries. They seem to work fine but not quite as good as the lead acid golf cart batteries.
http://www.priveye.com/van/
Roscoe