Unless your boat is at least 66' LOA, you don't need a Type II you can use a Type I. I recommend the Raritan PuraSan
Raritan PuraSan Promo Sheet Although it was designed for use on boats in fresh water or with toilets that designed to use onboard pressurized fresh water, it works equally well in both fresh and salt and unlike the ElectroScan, does not need salt.
I do not advise entirely replacing your Type III system to a Type I...you'll still need at least a small holding tank for use in "No discharge" waters where the discharge of treated waste is not allowed.
You might want to check out the link in my signature here...it'll take you to the current edition of my book. The title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie