It depends on whether you are mounting flat to the floor or have to mount to a raised black tank. I have found the Sea-Land Traveler series to be very well made and easily serviceable. They have a pedal flush mechanism, and you can add water to the bowl easily by lifting the pedal before flushing.
They are offered in white, bone and several other colors and MSRP is around $285. The 500 series is particularly popular.
Of course, if you want to go totally deluxe, you can get the Concerto residential style with electric flush or a Thetford Aria II for about $700, or you could even go with a Microphor or HeadHunter which will quickly get you up into 4 figures!!!
That's why they call some of them the 'throne'!