First and foremost, welcome to Expedition Portal. Glad you took the time to sign up and post. Your thread will get moved by a Mod at some point as the trailers this sub-forum refers to are an animal of different spots.
As for your inquiry....there's a lot to chew on there.
1) The truck camper you're talking about sounds like it will be pretty heavy because of the slideouts. It might be suitable for maintained gravel roads only however the brand you're after might be a lot sturdier than what I'm familiar with. Also, although your truck when outfitted with 4WD CAN be taken off road, you might find operating off road a truck of those dimensions and cost will get uncomfortable pretty quickly (in terms of handling, clearance and potential costs if damaged). Perhaps a smaller truck with a lighter camper seems like a disadvantage at first glance but your living space can be extended with a side door truck camper and an awning with an enclosure such as a Fiamma Florida Room. The maneuverability on more technical roads would be a relief over the F550, IMO.
2) What kind of boating do you intend to do? If your mental picture is towards the more remote, perhaps an inflatable boat (Zodiac, Avon, etc) is a good option as it will tow behind a truck a lot better if you truly intend to drag it behind you on unimproved roads. Not sure what level of comfort you desire / require but that might help to help you with some feedback.
Not sure what your budget is but if you're kitting up an F550 or an International Terrastar with 4WD, you might want to consider an EarthCruiser (
www.earthcruiser.com.au). No doubt you would have EVERYTHING you desire in terms of all-terrain mobility and year-round comforts. The manufacturer would also probably have buckets of ideas on what kind of boat will answer your needs.
Me? I'd go for the EarthCruiser in a New York minute and tow one of the higher-end Zodiacs (like a Bayrunner) on a high-clearance trailer.
At any rate, welcome to the forum and congratulations on the retirement chapter of life you're starting. Fun, fun !!