Your aircraft is an heavy one.
That may be the case. But the only other person I've known who has bought a Twin Com also got a nasty surprise when he got the real W&B sheet. The message is clear -- weigh before you buy.
You can store 30gl in he tips (180lbs) that can be kept out of your W&B.
That's a strange way of putting it, and numerically incorrect. The installation of tip tanks increases the MTWA from 3600 lb to 3725 lb, with the condition that any weight in excess of 3600 lb is fuel in the tip tanks. So in essence you can have 21 USG (125 lb) in the tip tanks without having any impact on the passenger and baggage load.