Pretty well every travel trailer manufacturer offers shorter models with most of what you want but where you'll likely be disappointed is with cargo carrying capacity which will usually be much less than you're asking for. My own Coachmen Freedom Express is a bit out of the norm as it's one of few models in the sub 20' category that offers well over a ton of CCC whereas most competitors models are half that. It's tank sizes however are 30 gals, typical in this size trailer, 'though I do know that Lance offers 45 gal tanks ... but 60 gal I doubt you'll find in many shorter travel trailers.
Regarding trailer size and campsite access it's not just length that can be a restricting factor but also the height of a 5th wheel and the reason some for whom access is a concern will opt for a travel trailer rather than a 5th. My own Freedom Express is just 22' 6" coupler to bumper and I can place it on pretty well any campsite and because it's not anywhere near as high as even the smallest 5th low hanging tree branches just haven't been an issue.