There are plenty of stuff thats available from B&Q for unblocking drains and sinks, i use one regular on the sink at work, its in a yellow bottle and costs around a fiver - pour a quarter of the bottle down the sink/plug hole and leave over night - always does the job
Has this just started to happen or has it always been there.?
Try pouring some bicarb down the sink and then pour some vinegar in on top of it. It fizzes like mad but usually cleans it.
Periodically pour some loo fliud down the sink to keep it smelling "*** lovely **
Try some net curtain wire, it will follow any bends but is rigid enough to break up the blockage or, use a dingy pump or similar, seal into the sink waste with a rubber glove and pump, it will clear the blockage or blow the pipe joints apart!
sounds like the pipe may have collapsed, disconnect from the tank and check it straight. From one side of the van to the other is very little drop so it does block easily, have you tried a sink plunger.