RV toilets are designed to flush with a minimal amount of water, and sometimes that water just isn't enough. That's when the hand-held hose is useful. Mine does not have it, and I sometimes have to take a wad of tissue in my hand and wipe the muck down the bowl. I don't mind -- I'm a grownup. But a hand nozzle would be useful.
(I'm considering giving the bowl a good polishing with car wax.)
I sprung for a Thetford Aria, and I don't regret it. (But I did not get the one with electric flushing. That's just silly. )
And the tall version, yes -- 18" at seat surface. Absolutely.
Full size oval seat, absolutely.
Practically all RV equipment is of lousy quality. Even my $500+ Aria. To install it, I had to either carve a divot in the floor for clearance, or modify the flush-mechanism to not scrape the floor. (I modified it, being the stubborn type. lol )
After a short time, the pedal would not return, and we had to lift it with a toe. I finally solved that by learning where to squirt silicone dry lube.
When you build the bathroom, I suggest you build it larger than you think necessary. Humans need more elbow room in real life situations than we tend to envision.
When you choose and install the "black" tank, go for a deeper tank rather than a wider one. And do not let the pipe from the toilet be longer than necessary. That is... have maximum clear space between the end of that pipe and the bottom of the tank. (But don't risk it coming out of the tank if the top of the tank can flex!)
The point is.... If you remain camped without moving the bus for a while, you may get a pile of muck under the pipe. I've had mine back up in the pipe, while most of the tank was still empty. (Broom handle, yes.)
Best of luck!