Judging by the fittings, you bought a rotocast polyethylene tank. The proper and best fittings for those tanks are installed by spin welding. Those compression style are notoriously iffy, but if your tank is below the floor maybe less worry to you. The seller certainly won't give you install advice because of this and wanting no liability for leaks and any ensuing damage.
Since you already have the tank, giving you contacts where you can order a tank to spec isn't going to help. What I would suggest, personally, is checking around a few RV shops, agriculture equipment repair/suppliers, and similar to see if anyone can spin weld proper fittings on for you. It can be done DIY after either making or buying a spin weld arbor to fit the fittings and using a high-speed drill. However, I don't recommend a crash course because you need to understand the plastic's melt behavior and when enough heat and pressure has been generated to weld. Otherwise, you can end up with a cold, brittle connection that cracks, or spin yourself right through the tank wall.
Sorry I don't have a more positive response for you.