The visitors buoys off the town were good if you have transport ashore. Cost I think was £11.30 a day for our 35 footer last week. Not too bad as there's secure parking for your tender and good free showers. Or try the Visitors Haven if you want a pontoon berth.
There are also a number of pontoons further upstream towards Malpas/Truro that run down the centre of the river, quite sheltered but I'm not sure of the cost. Down side is transport to and from them
The pontoons upriver of Turnaware are £8 per 24 hours - nothing about length of boat, so £8.
There's another pontoon, just below Malpas, that is within dinghy distance of the village. Above Malpas things dry out a bit!
There are a couple of livaboards on this pontoon, so someone to keep an eye on things. There is a bus service to Truro, and bus, train and National Express from the city.
The Port of Truro goes from Truro down to just past Mylor Yacht Harbour, and just above St. Just-in-Roseland. Below that is the Port of Falmouth, including Penryn and Flushing.