If you ain't got the truck or RV, the suggestion to drag it to a set of scales probably isn't realistic. Funtowngiddings.com has a tow guide which is actually slightly useful for those just looking to get an idea of truck and which trailer they can choose. I suspect tongue weight and the trucks load capacity are not figured in but a decent place to start estimating:
https://www.funtownrvgiddings.com/towing-guides/
I would then look for a truck that exceeded the minimums. In other words, if you choose a 10K lb trailer (gross weight) get a truck that is comfy with quite a bit more than that which will give you a more peaceful ride and a larger safety margin.