You need to use slightly larger waste tankage than fresh tankage. To figure your black tank size... Keep a log of how often you go to the bathroom per day for a month (both at home and away and for #1 & #2). For RV toilets, in order to keep from having a problem with toilet paper, you need to use plenty of water. I suggest a single flush for #1 and a double flush for #2. You are really counting the flushes. Find the toilet you want to use and see how much water it uses per flush. Take your month long flush numbers add them up and then divide that number by the number of days in your month to get a daily average. How many days do you want to go between dumps? Multiply the number of days between dumps by your average daily flush. Multiply the resulting number by the amount of water used to flush. Now add at least 15% to the number. Turn the number into gallons. Then refigure because you don't have enough space for a tank that size. If this is a weekender, you only need to go the length of your weekend. For longer, you may need to get a 4 wheeled side discharge tote (a bit pricey) to dump your black & grey tank into. Get a tote that will hold the entire contents of your black tank in one discharge. Trust me on this one. Been there, done that with a two wheeled tote. I still have the Barker 35 gallon two wheeled tote we used on the popup (cassette toilet so it was only used for grey water) and have used it on the Class C. I think I will let it go with the Class C when Das Mel sells it this Spring. It was very heavy and difficult for me to tip up on end to dump. I could do it but a side discharge would have been better and well worth the $$. Change the original wheels out to rubber ones (popular RV mod) and get a "float" to know when it's full. I had to drill a tiny hole in the top of my float so it would work right as it fit into the vent and without the hole it would not vent. This is a good basic thread to read about people who actually use them and why they use them. I suggest you read it to get an idea of how much tankage they are filling up.
On edit: to keep it cheap and easy, you might want to make your own "portable" waste tank from a black ABS rocket tank (warning the larger ones are baffled) and put it on a flat dolly or wagon
then just hook it up when you are in a site. ABS means you can add (glue in with all-purpose plumbing cement) your own fittings. You just need a space to carry it.