Due to the high diesel costs- I imagine it will be up to $10 by next winter, I hope to take the car to Texas instead of the truck. Because the truck has to have the block warmer plugged in, is it safe to use a timer, and just warm it for a few hours a day? If the tank is on 'Empty', can the truck sit for several months without any warmer?

 

So are you saying you won't be driving the truck at all next winter? If so, there's no need to plug it in.
fwiw, I've used a timer the past two years, but it's an outdoor rated unit that I got from Menards. Quite a few winter nights in SD don't require it to be running all night. Usually on for four hours before I leave is good, and I wouldn't be comfortable using the kind you would use for an indoor lamp.

 

As an electrician, instructor and inspector, what would suffice would be, for example, and Intermatic T101, with a NEMA 3 enclosure if you want to locate the timer outside. You can get them as an assembled unit rather than purchasing separately; your electrical supply retailer will know the part numbers, depending on the manufacturer it carries. Of course, you'll need to wire a receptacle in a weatherproof enclosure, with a weatherproof cover, nipped off the timer. Or, simply keep the timer indoors to avoid the weather damaging it. It's rated for 40 amps of resistive load, which is about three commercial block heaters (115V @1500W). There are other styles of timers, but the popularity of this one and its resultant price, and the durability and longevity inherent to its construction makes it a good choice. If you're only heating one truck, than the timer will last forever running the contacts at one third of it's rated load.

I also have mine set to come on four hours before I leave. In the extreme winter, it helps the engine. But mostly, it helps get the cab warmer sooner rather than the heater blowing past a coil of cold, sat-outside-over-night coolant. I don't see the point of keeping the truck heated if you're not driving it, unless your climate endangers the engine by exceeding the rating of your coolant mixture. But, I'm in the electrical profession and not the diesel profession, so I wouldn't really know.

Incidentally, if you have an electric water heater, and can manage your hot water usage times, a similar timer will help you manage your hot water electricity. If interested, I'll explain. Otherwise, I won't clutter this post.

 

I don't plug mine in until it the over night low gets down to or below 17 degrees. Below 17 an down to zero I set the timer for 1-2 hours, -5 to -10 degrees below zero 3 hours any colder then -10 below zero 4 hours max. Oh mine is parked a non insulated, non air tight garage so if yours is outside it may need longer time then what I use.