What tanks have you got- I am torn between, ronco and sealand...Sealand seems more expensive and not sure why?
The tank in my boat is original. Have not looked at Ronco or Sealand, but I would advise you to buy the one with the thickest walls you can find.
Where are the inlets/ outlets located? inlet from head should be at the top I beleive,
Yes - this is correct.
Is there any negatives to having the outlet at the bottom side wall going to the y valve from there ( after the Y is either the macerator or the pump out? If the outlet is on top..will the dip tune get clogged or leave a sediment in the bottom of the tank?
The tube could get clogged yes - not sure how likely that is if the tube is of sufficient diameter. There is a tube in our tank and it has not proven to be a problem. The thing to look for is a tank that allows you to pump every last drop of the stuff out - try to avoid buying one that would allow a few inches of sludge to remain.
Inspection port and vent outlets at the top I believe.
Yes. The larger the vent - the better. If there is sufficient oxygen getting to the tank it will not smell too much. If there is no oxygen getting in it can reek badly.
Good Luck !