The two best choices are the Raritan Atlantes and the Raritan SeaEra. The Atlantes is top of the line...and the closest thing to a household toilet made--full household size one-piece china bowl and pedestal, 4 flush options including "dry"...and if ordered with the optional internal vented loop will even hold water in the bowl "like the one at home." List is around $1100...discount prices in the $800s
If you're not ready to spend that much, the SeaEra is the best "bang for the buck" on the market...IMO a huge improvement on the Crown. It's also a basic macerating toilet with only one flush option, but MUCH quieter and uses less than half the flush water and power required by the Crown. If your bowl, seat and lid are still in good condition, you can save a few $$ by going with the conversion (everything BUT the bowl, seat and lid) instead of the complete toilet. Raritan even offers the SeaEra conversion as an alternative/upgrade to the Crown "lower unit" exchange.
Check both toilets out on the Raritan website at
http://www.raritaneng.com These folks have the best prices I've seen for both toilets (but you might want to check Raritan's exchange pricing against this price for a conversion):
http://www.boatfix.com/elec/rartoi.asp