Once it has frozen it is too late to add an antifreeze product. Rock salt might help but it would be difficult to get enough of it into the right place and your black tank drain line is most likely frozen as well and it will be a bigger challenge. If you could somehow block the air around it and possibly use a heat-lamp that might help.
Do you know that the entire tank is frozen or is it just that it will not empty?
If you have a wife with a hair dryer, she might hate you for this, but getting a hair dryer on the tank from the underside and heating the tank is probably your best bet. Move it around so you don't melt the tank.
I'd be super cautious about dumping anything in the tank. The only other thing I can think of is using the -20F washer fluid. It contains methanol which will work to some extent, but washer fluid is actually flammable. You really don't want flammable stuff in your tanks and methanol is very environmentally unfriendly, bad for septic systems.
I would NOT add hot water under any circumstances once the water freezes your tank will expand and burst.
I would go buy 5 or 6 jugs of RV ANTIFREEZE and dump it in, leave it alone dont do anything else for a couple of weeks and then see how solid it still is. Your too late to the party now its damage control
If I had to do it I would probably fabricate a shroud from cardboard box material, adapt a space heater with some dryer vent pipe, shroud the valves, lines and bottom of the tank (assuming they are close to each other) and crank on the heat for a few hours.
I got involved in a project about runway icing in my last couple of years in aerospace. I visited Alaska in the winter and a couple of other airports to talk to the people that do this.
Unanimously they agreed that it is not ice removal that is the issue. It's ice prevention. The guys in Alaska said that if they let an inch or so of ice form early in the season, it will be there for 6 months+ until the thaw.
Direct answer is that there's nothing you add to a tank to unfreeze it. Too many assumptions going on here to recommend a specific solution, namely what the goal is and the timeframe to get there. Ultimately the goal is to "empty" but is that because it's full to the brim and a sudden panic or just a routine dump. Does this need to happen now or just in time to go on a trip in a few weeks. Is there any consequence to only a partial dump to get by or must it be totally empty. All of those factors plus the facilities available to effect them are unknowns. If the OP comes back with more information then a targeted solution can be formulated.
I've run into an issue where the tank itself wasn't frozen, because I have tank warmers, but the 4 inches or so of drain pipe coming off the tank to the blade valve froze. I'd suggest trying some heat tape on this area to see if it can thaw put enough to open the blade valve.
I would introduce slow overnight heat to the bottom of the tank.
Small electric heater, use caution to were the heat is directed.
A second heater in the bathroom directed toward the toilet can’t hurt.
Simple table salt will help loosen any ice that may have formed, add directly with almost boiling water.
When you get it drained, flush tank with lots of water.