Howdy Mr. Trucker,
Since my gasoline car exploded over Lake Superior in 2002, I've been running an88 Ford F250 7.3l and a 76 Mercedes 300D on waste oil.
I imagine your freightliner has two fuel tanks already, probably linked in parallel?
Alls'you need are a couple of solenoid switch valves (I only know ones in 5/16 or 3/8 inch ID, but your truck probably has a larger ID fuel line, so you'll have to find the larger line. You may want to choose a larger ID for the tank you want to run on WVO.
I'm assuming you know the basics: one tank for starting on diesel/biodiesel, one tank for wvo. You'll probably want to have separate filters for diesel and wvo, because you don't want to stall a big rig like that at highway speeds!
You can make a cheapo heat exchanger/filter by tee-ing into your heater coolant and wrapping the hose around your cheapo-veggie oil filter (15 micron minimum; 10 better) and adding some sort of backup electric heater (two Vegtherm Megas?). And, come to think of it, you'd probably want a GOOD coolant heat exchanger, cuz if you're truckin' you probably go through lots of sudden temp changes (mountains, sudden cold fronts, etc). Check out Plantdrive.com, Greasel, etc).
And get a free meal from the truck stops for picking up their used cooking grease!
YOu also will want to add a temp gauge and pressure gauge for when you're running on veggie, so you know the oil is hot enough (150 F min) and the filter's not starting to clog (pressure gauge increase reading). Hope that helps.