use a piston pump with fully 'plastic poppit valves' .... benefit is that such is 'positive displacement' and will be able to pump suitably against a deep 'static head' volume of 'muck' in the tank. Youll need a 'diffuser stone' on the 'terminal end' of the air delivery hose.
You really dont need 'much' air volume to continually 'sweep' the noxious anaerobic gasses out of a holding tank and to keep it 'aerobic' ... a few/several cc's per minute of air will do.
The totally enclosed non vented (explosion proof) small 12vdc motor isnt going to be easily obtainable.

Just a large diameter vent line will do an almost equivalent ... most small diameter vents on boat holding tanks are simply 'too restrictive' to affect any gas 'mixing and diffusion'.