not knowing the material it does not look like rotocast plastic, I would take a small sample and go to a tractor supply or some such supply store and buy a like sample material plastic item. you may well be able to repair using a heat gun by welding. poke a hole in new test materal and see if your bay sample will weld to it. we actually have used clorox bottles to fix a leaky coleman conoe.
you can sand down the inside to make it smooth while adding girth to the bottom side.
look at https://www.polyvance.com/
an alternative is to use starboard and 5200 hull sealent, you can find at west marine online. I use it to mount switch boxes on the transom on my fishing boat. this is a none flexible fix, where weld is plyable.
I would replace the white sewer access plate as it looks cracked.