I am also considering getting a composting toilet for my 15A when the time comes.
There are newer generation composting toilets that are much smaller and less costly than the old gigantic, heavy Sunmar models that you may be familiar with. They don't generally require electricity to run. They are cheaper than the older models too...the Nature's Head and Air Head will run about $1000; the C-Head is more like $500.
Nature's Head Composting Toilets: Saving Our Water for Tomorrow
C-Head portable composting toilet system
The general idea of all these newer designs is that you purposely separate the solids from the liquids. The liquids go in a separate container that is emptied as needed. The solids go in their own bin that contains peat moss or coir (coconut husks) and is mixed to create a non-smelly product that only needs to be emptied after months of use.
The old giant Sunmar designs had everything mixed together and required electricity to run a fan to evaporate off all the liquid. When you deliberately separate the liquids from the solids, that is not necessary.
These newer types of composting toilets are highly thought of by folks in the boating world. There, they eliminate the need for holding tanks, blowers, and through-hull fittings. I figure there are similar benefits in an RV application: no holding tank, no dragging the trailer somewhere to drain the tank, less water needed when boondocking as the need to flush is eliminated.
Here's a short video on the use and benefits of a composting toilet installed in a Class A motorhome --
The Winns talk about a computer fan to help with evaporation and odor control, but that feature is optional.
As far as fitting in the 5.0, that is of course a question for the folks at Escape. In my case, I'm planning to put the toilet in the driver's side front dinette seat. I'm leaning towards the C-Head which requires 18 inches of height clearance -- the dinette seats in a 15A would need to be raised a couple of inches to accommodate that height. Since that area is not fiberglassed, I'm thinking that is a change that Reace could do strictly with the cabinetry and floor level there.
That way, I preserve two sleeping/dining areas, keep the front window, but still have a place to 'go' when I need it.