You are correct that your old trailer will fit a lot of places that a much bigger 5th wheel won't. Obviously the length is less, but height is a major consideration too. Lots of great national forest campground or dispersed campsites will have plenty of length, but trees overhanging that are a hazard to 5th wheels.
Your trailer is already depreciated out. So far as costing you money to maintain, it will of course need the basics like tires and insurance, plus caulk for maintaining it leak-free. What you choose to spend on it otherwise is totally up to you. You will not need a lifted suspension to get into MOST national forest campgrounds.