You really have to weigh it, don't guess. It can be a little intimidating the first time but isn't that big a deal. Go to your local truck stop, talk to the clerk first so you understand how things work and go for it. At a minimum you need front and axle weights. Ideally you need weights for each corner.
There are other places like sand and gravel dealers, grain elevators, etc. that have scales but truck stops are the easiest to find. If the scales look busy, find out from the clerk when you should come back, you don't want to hold up the professionals.
You should weigh with a full tank of gas, full fresh water tank and normal load of stuff. Ideally you'd also have a passenger with you but that's not a big deal.
Secondly, there are weight/pressure tables for your specific tires. These are the
minumum pressures for the weights. If you get the four corner weights, all wheels on an axle need to be inflated according to the highest weight on that axle. The general recommendation is to add 10% to the minimum PSI for each axle.
Here's the best collection of tire info I've found (by a retired tire engineer). The link is to a write-up on pressure, there's a menu of many other articles, including weighing, as well:
RV Tire Safety: Cold Inflation Pressure
Here's a link from Goodyear that has some good info:
https://www.goodyearrvtires.com/
There are numerous posts on this and other forums on this topic, that go on for pages and pages. As with many topics, they can be long on opinion and short on facts. IMHO, the two links I've given you contain pretty much all you need to know.
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