We carried a 35 gal. for the first couple of years we had our TM. Only used it twice... and didn't really need to as we could have dumped at the dump station on the way out. Since our tow vehicle was a p/u, we carried the tank in the bed.
The greatest potential problem is the 6.2 gal. capacity of the "black tank." If you camp more than 3 days, and only use the TM toilet, 2 people will fill it. Suggest using the campground facilities as often as possible. We did and could stretch our "dump times" to 7 days. :
If you "boondock" then you want a blue tote that is large enough to hold all your gray water. First dump the black tank into the blue tote, and flush the line with a small amount of gray water. After dumping the tote, then come back and empty your gray tank. :P
Using a 5 gal. or 2 hooked together will not work, in my opinion, because your black tank holds more than 5 gal. and your gray tank holds more than 10 gal. :-/
Best solution, if you're set on buying a tank, is to get one large enough to hold the waste of your largest tank and that has wheels you can pull easily. Remember water weighs 8.3# per gallon, so the totes can get quite heavy when you are pulling them to the dump station. ;D