Don't go to Fowey without the RIB. Fowey is boating heaven!
There are two visitor pontoons on the Fowey side of the estuary at 1 and 2 on the attached map. There is a two hour max stay on the outside of the pontoons, which you should try to observe because they can get busy and it's a real inconvenience when people hog them! Also there are always a couple of harbour patrol boats in operation and they can be a bit strict. Smaller boats can use the inside of the pontoons, but I am not sure what the limits are there (boats can definitely stay longer than 2 hours on the inside).
There are visitor moorings all over the place but they are always afloat, so you need a tender to get to them.
All the above operated by Fowey Harbour Commissioners, phone 01726 832471, http://www.foweyharbour.co.uk/
There is some sort of boat yard near 3 on the map, which may be an option, but I do not have any contact details. I'm sure Fowey Harbour Commissioners could advise.
Alternatively, about a mile up the road marked 4 is Penmarlam campsite, phone 01726 870088, which has a lane down to the Penmarlam Quay and pontoon. The quay is again run by the harbour commission but the campsite are agents, so you can pay for everything at the site office. It's a nice informal facility with a good slipway and space to park the car and trailer, but it does get busy in high season. Launch and car parking for a day is about £8, launch and car park with overnight boat storage on trailer is about £10 if they have space, or keeping the boat on the pontoon is about £20 per night. Only problem is you will need to take the car on the ferry across from Fowey (thats the dotted line on the map) to get to the campsite! The ferry is about £3 each way.
The slipway you looked at by the ferry and the one at Penmarlam are both usable at all states of tide.
When is your holiday? I'm down that way a lot in the Summer so might see you there