Check out the Raritan SeaEra QC
Raritan SeaEra QC available in both sea water and pressuized fresh water versions...either as a "conversion" (everything south of the bowl) that will let you re-use your existing bowl, seat and lid (yes, your bowl WILL fit!). I'd go with fresh water version...lower power consumption, less flush water, and MUCH quieter than any sea water electric toilet. And, with a pressurized flush water toilet, you'd be able to use one of the upgraded flush options that let you bring water in ahead of use, have a "dry" flush, and a low water flush...conserving your holding tank space.
Measure your space, keeping in mind that the pump can be oriented in more than one direction...if it won't fit, we can look at other options.
Get a quote from the online store here...they usually have excellent pricing on Raritan products. And I'll be glad to help you sort out any issues with the installation.
Installation isn't that hard to do. You'll need to run a new dedicated circuit for it, on its own breaker, making sure to use the right wire size for the distance from your battery, but your existing head discharge line should be useable.