The novel I'm writing next is set in a love hotel, so I wanted to actually see one for myself.
Literal
Next-time write novel [in-で] love-hotel [subject-が] stage become [reason-んで], once actually see-want[explanatory-んです].
舞台になる 'to become the stage' is the standard expression for a story's setting. ~ておきたかった uses the preparatory ~ておく ('to do in advance') in past-tense desiderative — 'I wanted to have seen it ahead of time.' 一度実際に emphasizes firsthand experience over secondhand knowledge. A fun excuse for a love hotel visit.