At one time RV toilets had an overflow tube that would bypass the flapper valve, The tube had an S shape to act as a trap to prevent odors from coming up. With that arrangement one needed to fill the toilet from time to time to replenish the water in the overflow trap. Now the genius toilet makers eliminated that simple thing to save a few pennies. Of course a running toilet would eventually overfill the black tank, but you had a good chance of heading off a big problem.
I installed a Dometic 310 china toilet, it is the same as the elongated seat 320 model. Well that thing cracked the flapper and there is no replacement part, you simply toss the toilet. I wouldn't recommend it for that reason and the fact that it also wastes water.
The suggestion of running off the pump is probably the only reasonably safe way to use a modern cheap RV.
As far as clamping hose and PEX fittings, KZ and probably many brands simply attach flexible hose to PEX fittings and that is wrong. Hose attaches to a hose barb and PEX attaches to a PEX barb.