Well I presently have a 1977 F-250 2WDR super cab, C6 auto trans, 400CI (6.6 litre) 3.73 rear end.
And only tow a 1989 18.5 Prowler Lynx fifth wheel, but am thinking of a used 1988 25 foot Okanagan fifth wheel, will I still be good with this truck, or is it a good thing I am looking in the local buy and sell to see if there is something better out there?
I am happy with my old truck, but if I am taxing it too much then I would rather get something that will give me a margin of safety.

The 400CI is probably enough motor even in the local hills. However, you might bump up against a load capacity on the truck with that trailer. If possible, try and find out what the hitch weight actually is, Not what the factory says it is. Then you can try and figure out the max load capacity of your truck after it is loaded to go camping, hitch, people, fuel, fire wood etc.