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 cleaned the cracked area with a scotch-brite pad and not sand paper as I didn’t want to close the cracks,
I taped over the gray tank vent, taped a wet-dry vacuum cleaner hose to one drain and had a helper cover the other drain with her hand when commanded.
I used the vacuum cleaner to create suction and used the syringe to apply the ABS cement to the cracked area; apparently an adequate amount of cement was drawn into the cracks to seal them.
Before applying the cement, I slightly heated the cracked area with a hair dryer to help ABS cement to be drawn into the cracks.
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/B07B93YKX2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
For your application, you could plug all drains, open the drain valve and have a helper stick the vacuum cleaner hose into the drain and use a hand to seal around the hose. This allows a vacuum to be applied for only the short time while you are applying the cement. You don’t need, or really want, an airtight seal for the vacuum cleaner hose.
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.