I don't drive my RAM diesel pickup very often, so I want a battery charger to get it started at times. What would be a good charger? This is a single battery 3/4 ton pickup.

 

You'll need a massive amount of current to get that thing started, hundreds of amps. Few chargers are capable of that kind of amperage, and cost a lot. You could consider keeping the vehicle battery in the garage and on a trickle charger, installing the battery each time you want to use the truck. If that's too much of a PITA, you could consider keeping another battery on a trickle charger, and wheeling it out to jump start the truck. There are trickle chargers that you could wire to the truck, if you have a suitable outlet that's protected from the weather at a reasonable distance from the truck.
If you know in advance when you are going to need the truck, an overnight charge with about a 6 amp charger should be enough to get it started. A battery that is left to run down to dead frequently and then charged up will be pretty well shot fairly quickly. Lead-acid batteries last much longer if kept charged.