I'll agree and just echo what everyone else has told ya. Sounds like your 1500 is doing everything you want it to and has exceptionally low mileage. I service both 1500s and 1800s and probably the biggest advantage of the 1800 trike is its fuel injection, but really, if your 1500 is running fine, I'd just go ahead and keep it....wear it out...LOL.
I will say this: in their 2-wheel versions, I personally found the 1500 to be the better riding highway bike, but it's heavy.
The 1800 brought fuel injection, easier rear tire service, lighter & nimble. Other than fuel injection, none of those "advantages" apply to the trike version. Yeah. I'd just keep it and wear it out.