Dometic is a solid brand, and their newer 320 would be a good replacement. I’m pretty certain the bolt pattern (just 2 bolts), and the water inlet is exactly the same, so replacing it should be straightforward.
No matter what toilet you get though, 320 included, check the toilet spec sheet and make sure you have proper clearance to the back wall. The 300 requires a distance of 8.5 inches, while the 320 requires 11 inches, I believe. SO measure first to make sure it will fit.