Several makers provide caravan/mothrhome water traps that can be integrated into the sink outlet and there are other options for inline use.
Reich is the specific brand that was factory fitted on the two Hymer caravans we have owned. This features a half turn screw off base to clear if & when required.
This is an example, I expect they are also available for different sized outlets
LINK. Plus for inline use
LINK.
Note these are pulled off the web for illustration purposes only and no recommendation of these particular vendors.
The use of a loop in the typical corrugated pipe coupled with that's inherent difficulty to open and clean, is a very poor, though cheap solution to providing a trap. IMO reflects on the van maker's standards.