Open the drivers door. There should be a yellow tag on the truck door frame. It will tell you what the payload capacity is. This is where a lot of people get overloaded with a 5th wheel. Part of that payload capacity is the weight of the hitch, passengers,, and anything else in the truck. There will also be a tag showing axle capacities, never exceed those.
There should be a tag on the 5th wheel, probably drivers side, that shows Gross Vehicle Weight Rating.There may be a smaller yellow tag on the entrance door of the 5th with cargo capacity.
If you are looking at RV manufacturer webites for weights, they may be a bit misleading. If the dealer adds a bedroom A/C, extra battery, etc. that is usually not included in published weights. My Montana HC was listed as 2175# pin weight. By the time I installed the hitch in the truck and added cargo I was about 3500# pin weight on my truck. I was about 300# overweight on a F3550 SRW diesel and traded the Ford for Ram with 4100# payload.
I previously had a Cougar XLite 28SGS and pulled it with a GMC 2500 6.0 Short Bed 4x4 with a 3.73 axle. It was barely OK in flat land but after the first trip to the Tennessee hills I traded it for the F350.