Why not forget holding tanks and just use portable tanks?

That would work for most boondocking areas but most RV, National and state parks require RVs to meet RVIA standards which include holding tanks. You'll see park rules state "Must be fully self contained"

Recreation Vehicle Industry Association (RVIA)

Those blue portable tanks can be a hand full to dump depending on where you have to dump them. The smell can be deadly.

Holding tanks with a sealed sewer hose setup is the best way to go. Little or no smell or mess.