I am looking at cutting the elbow and hose fitting then attaching a PVC valve of some sort onto the 3 inches of PVC pipe. Its enough pipe to glue in a valve.
You don't need a valve....There's an easier way: leave the elbow and connect a length of PVC sched 80 to it that has a 1.5" OD long enough to reach the top of the tank. Switch to 1.5" hose to go from the top of the pipe to the macerator pump
. Done.You'd have to heel over almost far enough to call it knockdown for waste to run past the top of the pipe.
Even easier if you have enough clearance above the tank: Install a new discharge fitting in the top of the tank --actually pretty easy thanks to a li'l gadget called the Uniseal UNISEAL
--and install a pickup tube in it that goes to the bottom of tank. Use PVC (sch 40 will do for this) that has a 1.5" OD long enough to connect a hose to it, and just touch the bottom, which should be cut at about a 25 degree angle (any flatter, stuff can get trapped under it, any steeper leaves too much in the tank). Use a threaded plug
wrapped in plenty of teflon tape, both of which you can find in the plumbing
department at any decent hardware
store, to seal the existing discharge fitting. No more waste standing in any piping.