If you already haven't checked the price of tanks, you will see some tanks can get fairly expensive and installing them where they don't throw the the whole weight dymanics of your camper off can be trouble. I would never convert the fresh water tank to gray because the day may come when you upgrade and then you will have to replace the tank to sell it, not alone doing a lot of re-plumbing. I would go to the local camper store or online and purchase a 25-30 gallon Barker Blue Tote holding tank. It may be your best bet. I have one and they are great and can take several days to fill, of course depending on how many people are using it.
Good Luck in your decisiopn.
2016 Forest River Surveyor 201RBS
2014 Chevy Silverado 1500 4.3L