What size tote?

We pick up our new 3124kb in two weeks so we're beginning to accumulate all the stuff that doesn't come with it. Amongst those is going to have to be a portable grey/black water "tote" since we are often in locations where it is electric only and need to haul our waste over to a dumping station.

Question is: How big of a tote should we get. I see that Thetford's come in 12, 18, and 27 gallon sizes. Immediate reaction is to get the 27 since it is similar in size to the TM holding tank but then, I thought, "There's a reason these come in smaller sizes!" I know that water is 6 pounds per gallon and assume that waste is similar. Is weight a factor I should consider or should I just get the 27?

What about storage when travelling? Where do you put these things?