As always, thanks to everyone for the informative responses!
Cost aside, using a macerator would be a no-brainer. Never have to move the tank and just let the macerator do all the work! But, hey, probably talking about $375-$450 (Amazon prices) for the macerator and tank. Unfortunately, we won't be dry camping enough to feel comfortable making that kind of investment.
Definitely get the impression it's a big deal to NOT overfill with the black tank--for obvious reasons. Easy to see how constantly keeping tabs on the black tank level could take away from the dry camping experience if I were to get a waste tank smaller than my black tank. Not so worried about overfilling a waste tank with my larger grey water tank. A little soapy water ain't gonna do me in (FWIW, kitchen sink goes to my black tank in our TT).
Anyhow, after reading everybody's responses, I think the
25 gallon Tote-N-Store for $130 (with free shipping) is a good all around compromise (cost & functionality). Received good 4-5 star reviews on Amazon and has a pretty low price. My black tank is rated at 25 gallons (actually a tad less due to air pockets), so I shouldn't have any overfill issues. Still have the option of only doing partial fills if weight or trip length (to the dump site) becomes an issue. My grey tank is 38 gallons, so there will be some compromise here. Kinda get the impression it's definitely not worth the cost, or the sheer physical hassle, to consider a larger, very heavy, 40 gallon waste tote just to accommodate the grey tank, unless we plan on doing some serious dry camping.
Thanks again everybody!