If you stored the trailer with the black tank full, then it will be kind of hard, especially if you don't have anywhere to dump the contents. Can you tow the trailer around--if so, then put some Dawn and Calgon in the tank, along with some water and drive it around, (possibly add about 10lbs ice to it also) then find a dump station to dump it. While at the dump station, run tons of fresh water thru it, then leave about 5-10 gal in it, more Dawn/Calgon and take it home. Do the rest of your remodel, and probably it will be ready to use--unless there is a large amount of 'stuff' in it to start.
If you don't have the ability to dump at the house, you are going to have to take it to a dump station or use the old 5-gal pail method over to a sewer cleanout. Not a fun job, but it can be done...
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died