It sounds like a packed blockage to me.

Are you going into the holding tank -- or are you set for "offshore" direct discharge?

If you're discharging out, then go to the vented loop, and see if the blockage is at the loop, before or after it. Gravity is good. If it's before, then I'd first try a wet vac with some hot water -- first a little hot water, then some wet vac, then some hot water, then some wet vac. etc.

I would be very careful with a snake. You could damage the hose (certainly negatively affect is odor resistance).

If you're not up to it, There are some jobs that are best left to the yard staff, Doug. For the cost of dinner for two, you might have a lot more enjoyable afternoon. ;^)))

I have often sent a very unhappy few hours, only to find-out the worst: I couldn't fix the problem AND went through the shit, only to fail :^))).