Eat low fiber, near liquid diet and supplement with laxatives.
Use public bathrooms for solid deposits. (will also solve odor complaints)
Find a species of cave fish known to like poop and establish them in the holding tank.
Drop an M-80 in the toilet periodically.
Dig a hole in ground directly under toilet. Cut hole in holding tank above the hole in ground, Save the dirt from the hole in ground and replace it prior to departure.
Check out the results of this amazon search:
I am confident these will also work with homo sapien waste.
If none of these work, check back with me, I will supply more solutions..