Depends on type of plastic
I'm currently agitating over the repair of my black water tank. The bathroom floor rotted out, and the porcelain toilet settled down, and cracked the tank around the opening. Inland RV does not list a new tank for my trailer, but if they did, it looks like it would be around $500, so I have good incentive to repair mine.
The tank appears to be made of white ABS plastic. I was able to find a small piece of black ABS for repairs, but it is thicker than the tank's walls. Through age, and the weight of the toilet, the top of the tank, which started out flat, has bowed down a bit.
I'm considering several schemes of repair:
1) Grind the cracks into a "V" shape, and fill the V with a slurry of ABS shavings mixed with ABS solvent weld liquid.
2) Don't grind the cracks, and simply solvent weld the new ABS over the cracks.
3) See if someone at a radiator shop (they work with plastic radiators now) can heat weld it.
All three schemes are fraught with peril. I would probably be grinding the V with a hand held Dremel tool. There would be little Vs, big Vs, and gaping holes. Recipe for ABS slurry? Trial & error. Scheme 2 would build up the surface height of the tank. Schemes 1 & 3 might. Scheme 3 depends on me trusting someone I don't know working on something they might have never worked on before.
Pardon MY problems above - I just needed to think out loud so to speak. What I was going to share with you is this: There are a world of plastics out there - ABS, PVC, acrylic, poly-fill-in-the-blank, etc. Most plastics are too slippery for a glued on patch type of repair. The patch will pop off in time. The only successful repair, depending on the type of plastic, will be either solvent weld or heat weld.
It's easy to determine which type of repair you will need. Wipe a small amount of acetone on an inconspicuous place on the tank. If the tank gets sticky, it can be solvent welded. If it only gets cleaner, then you will have to heat weld it. I understand the pros use a heat gun instead of soldering iron type of tool for this type of repair. The trick is locating filler material.
I hope something above helps you with your tank.