Sink has a drain line with a 'p' trap and then goes to grey tank....just like a home
Remove the 'p' trap and clean out the drain line. You can use a snake down drain line as it is the same type of pipe as home.
Be careful using ANY drain cleaners....won't hurt the piping but could damage the dump valve seals causing them to leak.
Cleaning the grey tank.....
After you get the sink drain line/'p' trap cleaned out pour hot/soapy water down sink into grey tank (several buckets) then take truck for a drive.
Dump tank when you get back.