Backed up, spent small fortune so far
My 2cents. Call a professional plumber. These guys have seen a lot. They may be able to power snake it through the vent. If you get , don't do RVs, call the next one on the list. Got nothing to lose and at the least, may be able do evac the tank through the vent or toilet. Or perhaps a septic service to clear and pump you out.
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