I had a cracked pipe in the gray water system that I repaired using a syringe and ABS cement over 5 years ago and it has held up well, perhaps it will work here. I used vacuum pressure to pull ABS cement into the cracks.
I cleaned the cracked area with a scotch-brite pad and not sand paper as I didn’t want to close the cracks.
Cover the black tank vent line with tape or plastic so you can hold a vacuum pressure in the drain line.
Have a helper hand-hold a vacuum cleaner hose in the black tank drain and cover most of the space around the hose with a gloved hand when commanded. This creates a vacuum in the drain line and black water tank. You don’t need, or really want, an airtight seal for the vacuum cleaner hose.
Before applying the vacuum pressure and cement, slightly heat the cracked area with a hair dryer to help the ABS cement to be drawn into the cracks
While the helper is applying suction, use a syringe to apply the ABS cement to the cracked area; hopefully some cement will be pulled into the crack. Keep applying cement while your helper stops applying vacuum pressure, this is to keep some of cement from being pulled all the way through the crack.
I used a syringe with a blunt needle like this to squirt the cement in the cracked area while vacuum was applied. I used the largest diameter needle. I bent a long needle to get to cracks I could not reach directly. I applied enough cement to have extra material on the outside of the cracked area, and put down a drop cloth to catch drips.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It was a bit messy, and doesn’t look great, but who looks under a trailer anyway? I don’t think that anything bad would result if you tried and it didn’t work.
I never used flex seal, but a coating of that sounds like a good idea.