That "collar" is the female threaded fitting into which the male thread-barb hose fitting goes. If you look hard at the photo you'll see a lot of hard glop around it. That's not sealant, it's melted polyethylene. Fittings are installed PE tanks using a process called spin welding. A hole that's about twice the diameter of the fitting is cut into the tank, then a spin welder tool melts both the edges of the fitting and the edges of the hole while injecting new melted PE so it all blends together into a single mass. It requires a bit of skill that's only gained with practice.
If the collar is cracked, it's not repairable. Someone who knows what he's doing will have to replace it. Fortunately it's a water tank, not a waste tank 'cuz no one will work on a used waste tank! If there are any plastics fabricators in your neck of the Canadian woods AND you can remove the tank to take it to them, that would be the easiest least expensive solution. It would be worth a few hours drive to find one if necessary 'cuz the alternative is a new tank unless you want to try "strapping" the crack closed and wrapping it tightly in a lot of teflon tape that will eventually leak again.
And btw... Tank fittings are NPT standard, which is slightly tapered--so slightly that the taper in a female fitting that short isn't visible to the naked eye. Over-tightening the the male thread-barb fitting is just about guaranteed to crack the "collar," though it may not happen immediately.
Season began well enough and than after a good wind noticed some fresh water on my cabin floor. Made the assumption that the O-ring on the access port on my water tank was perhaps failing me or I had not put enough elbow grease into the tightening. Have since found this crack in the fitting on the tank. Wonder if anyone knows if this is a removable fitting or moulded to the tank. I have not tampered with it yet as I may need to use a sealant on the crack before it gets any larger.
