I wouldn't do either one until you've check the tank vent for the blockage I'd bet real money is the source of your problem. The inlet fitting on the tank is at the top, and last I heard, waste can't jump...so unless your tank is full to overflowing, what you're seeing is not a backup from the tank, it's waste in the head discharge line that can't get TO the tank....'cuz when air in a tank that's displaced by incoming waste can't escape, the tank becomes pressurized, creating back pressure that pushes flushes back where they came from.
The vent thru-hull is the most common location for a vent blockage...use a screwdriver blade, ice pick, whatever works--including knocking out any screen (screens create more problems than they solve)--to clean it out. Next, the other end of the vent line, both that end of the hose and the fitting on the tank...remove the vent line from the tank (I suggest you open the tank pumpout fitting to relieve any pressure before removing any hoses) and clean the glop out of both.
The archives of the sanitation and plumbing forum are filled with information about how to maintain a holding tank and vent system...you might want to spend a little time reading some of it.
