Slow draining water pipes

Annoying isn't it.

Caravans often has long runs of small bore horizontal drains that are not conducive to fast draining. 

You might be wrong in thinking that the slow draining is actually because the pipes are blocked. Caravan and motorhome drain pipes are often of relatively narrow size compared to home and usually have to thread round various parts of the unit to get to either the waste tank or outside waste container. We don't have a problem with our main sink in our motorhome but the bathroom hand basin is often slow to drain. I have been known to cup my hand over the plug hole to create a bit of suction and this often works. I would be cautious  of using household cleaning products in a motorhome for the use you intend just incase the plumbing is not robust enough  to stand up to the chemical. 

David

One of the problems is that some manufacturers use flexible ribbed plastic pipes from the sinks to the outlets which are run via some very suspect routes rather than strait to the oulet

in many vans, low set external underslung tanks mean theres little fall between the hole in the tank and the actual exit point.

little consideration as to how you're actually going to get rid of the water and hopeless tiny bore pipework doesnt help.

most installations look like the waste water pipework was a complete afterthought.

some do it far better, but why should only some vans be able to drain a waste tank sensibly?

poor design, lack of interest, poor components are three reasons.

I think that the OP is talking about sink drainage rather than tank drainage? 

in that case, it will still be poor design, lack of interest and poor components

I would have a look under the van and trace the pipe work and see if you can improve the routing. On one of my vans I changed the pipe work completely because of this problem and did manage to greatly improve the flow..

peedee

Could be a combination of things. Some prevention is better than trying to cure, we always use a wash up bowl and don't let wash up water go into tank, throw it straight away. This helps with even tiny bits of food and any fatty deposits. White vinegar down pipes, or rolla cola also helps remove some of crud from pipes. But take a look at some of other options suggested as well. Long pipe runs and intersecting pipes don't help in modern vans. 

It's because of what you have now agreed and  posted, it is costing Leisure companies  huge sums of money to "help" the problem by having to modify waste disposal points for their visitors which will have to be played for by everyone

it costs no more to place a waste disposal point in a convenient position than to place it in an inaccessible one....

poor MHSP's are nothing to do with MH designers.

your hobby horse has bolted