Buy an off-the-shelf one with a diverter. We've used both AirHead and Nature's Head composting toilets. We had the AirHead in our truck camper, and now have the Nature's Head in our class A. They both have pros and cons, but they are close enough that I'd just choose one based on the fitment in the existing bathroom/ toilet space you have. Both are leagues ahead of most homemade options, even though they're essentially glorified buckets with clever and convenient features added on.
Both use simple mounting brackets you screw through the floor, and a small hose you run through an egress wall or floor. You have to cap off your existing water line. There should be nothing specific to your Forest River that you need to consider, as long as your bathroom is located in an easy location to run the exhaust hose and 12V (for the fan) to.
We know several people who use them and none of them regret their decision. We sure don't - they're a game changer for dry camping as you aren't wasting freshwater to flush, and you can reclaim your black tank as an additional grey tank.
Both of the toilets mentioned have a ~2gal liquids tank that needs to be emptied every day or two with full-time couple's use, and a solids tank that needs to be emptied about once a month. I've found that the liquids smell far worse than the solids, though neither smell at all when the toilet is assembled. Regarding the post about tossing the solids in the trash - you end up with a solids bin that isn't runny or messy at all - at least in our experience - and is easy to transition into a couple trash bags (we double bag, just in case) without any mess.
