Older camper vans used toilets with 10 gallon holding tanks that toilet seat mounted on top of. Still available from some major RV toilet suppliers by $$$.That is a shame about the black tank rule (though i get the reasoning for it - people can be douche canoes) but I was thinking of a carrot trailer build and just using one of those Laveo toilets that dont need a black tank, but sounds like that would cause issues with the LTVA folks
Search the RV junk yard dealers for a used one. You will have to add the drain outlet under the van. For a sink waste water you can purchase plastic tanks sold to food trucks. Those can mount under a sink with a an exterior faucet outlet you screw a drain hose to.
Where there is a will there is a way.