Diesel storage tank question

Hi All..... I just acquired a 300 gal. diesel tank mounted on a tandum trailer. I intend to use it for fueling the tractor, generator some other stuff. Now here is the catch. There is a small amount of diesel in the tank, but the tank has been in outdoor storage for a few years, I recognize the remaining diesel is bad and should be drained. What should I do to the tank, if anything, before I add fresh fuel?

Thanks for the help

I think make sure you have all the old diesel & "crud" that's accumulated in the bottom out, & flush with a small amount of fresh. I'm assuming you have already checked to make sure there's no leaks or thin spots that's about to start leaking?
OH YEA, & don't forget a good filter on your pump.

Are you sure the diesel in it is bad? Diesel, especially if treated can last many years.

What I'd propose:
Get a good filtering funnel (I.e. Mr Funnel) and enough buckets/cans to hold as much fuel as you believe is in the tank.
Hook the trailer/tank to a vehicle and take it down a bumpy road for awhile (let's say a mile).
As soon as you return and stop (so any sediment is still floating in the fuel), drain the fuel through the funnel and put in the cans.
Re-seal you drain, pour those cans of fuel back in and repeat the road-trip/drain/strain process. Do this as many times as you feel you need based on the junk coming out. Once it's running without sediment, you're good to go to put new fuel in. And I agree with redneck. Put a filter in.

Oh, if you've already taken it off the trailer, just pick it up with the tractor and shake the tank around.