Tips for maintaining a third vechile as a tow only

I let mine sit pretty much as is until needed. No monthly excercise etc. I put a shade in the windshield, lock the door and walk away. Fuel can be empty or full as I make no special effort. The solar power on the trailer maintains the truck battery through a trik-l-start combiner. All maintenance is done by miles. No extra maintenance by time but I may adjust that some.

I agree with getting a full frame truck not a unibody.