Depending on the age of the van it should either be in the locker next to the water heater (a big yellow lever) or on the outside just underneath the heater vent (a screw out bung about 30mm dia).
I have Avondale Dart 2000 510/5. On mine the drain is external, as described above, the screw out bung is at the bottom left of the heater housing. There may also be a quick release valve at the top left of the housing. These do become very brittle and have a tendency to snap. Mine snapped years ago Fortunately I didn't have to remove I just kept the broken section in place and rely on slow drain only - it takes 2 to three hours for mine to drain. It is possible to refit the quick release drain but it can be a tricky procedure to remove without causing damage. I also change the seal on my screw out drain each year.
Geoff48 that top vent screw is available called a safety valve, I replaced mine last year. worst bit was getting the remain f the plastic fitting out, but being careful an taking my time it was ready for the new one to be screwed in, took about 30 minutes all done.