There are two sizes of Aux tank. We use the smaller which holds 340 lbs. I would suggest you look VERY closly at your C of G when using the Aux tank. we are limited on AUW and a very aft C of G with the tank in and fuelled. I would guess with the larger tank you won't have any space for baggage anyway!
There is a RFM supplemant for it. I'll see if I can email you a copy of the one we have.