We had a Dometic Aqua Magic with the flush handle on the side.
http://www.campingworld.com/shopping/item/aqua-magic-style-ii-high-profile-white/35653
Worked well until my wifes foot slipped off and it slammed up one time and broke some tabs on the flush valve in the mechanism on the toilet. Got replacement parts and when we I tried to put it back together, I ran into the same problem as the OP had. Finally gave up on getting it to actually stay together as the handle just pops on and off and we were camping and need a toilet.
Picked up this one with a lower true foot flush and it works great and is much more solid of a unit.
http://www.campingworld.com/shopping/item/aqua-magic-v-toilet-high-profile-foot-flush-parchment/26164