Grey tank stopped up won't drain

There is a hardware item called Power Plumber that shoots gas under pressure down the drain from the sink. Normally used to flush out hair, etc. but may open up enough of a path to keep you going until the degreaser works.
How about a plumbers snake? The clog might be at the P-trap. Can you remove the trap, clean it out and run the snake down the piping past the trap and/or run the snake uphill from the sewer outlet?