There are Napoleonic forts overlooking the Borstal area, but I think they were sold to nutters who wanted to build private dwellings in them. There is Fort Amhurst.

If you have a dingy with an outboard there is Dead Man's Island for tenting, but be careful of the currents.

THere is some scrub land the Strood side of the river, behind Morgan's timberyard and past the moorings - and if you pitch under the trees you might stay hidden for the duration, but I would not fancy your chances of staggering there from the GI without putting your foot in something - possibly the Medway.