Yeah, its impossible to tell how heavy a vehicle is by looking at it (unless you've been doing it a long, long time). The only way to know for sure is to weigh it. Its just like the garage door spring, it never gives any indication of how much tension its under until you pop it loose ;-).
I've been towing for years, and its amazing how fast pots and pans, food, clothes, water in the tanks, bicycles, backpacks all add up to serious weight. Plus how poorly the weight can be distributed making tongue weight climb faster than you think. I have towed way above my maximum payload/GAWR and etc. before. You're truck won't blow up, or catch fire if your overweight (well not generally). What will happen is you won't tow level (probably nose high), which reduces steer axel weight, giving people that coveted "white knuckle" feeling over bumps when the steering goes lite. Nose high/unlevel towing will also throw your headlights out of alignment, making it hard for everyone to see at night (you'll illuminate the sky, and oncoming traffic's eyes). It will also accelerate the wear on suspension components on your truck (shocks and bushings in particular go fast). It may also make your ride worse depending on how close to the bumpstops you come. Finally it can cause blow outs on your 5th wheel if its also not level and you overload the front axle/tires becasue you aren't towing level. See a theme here, towing level is key ;-)
Towing this 5th wheel will require due diligence on your part, to load things properly and carefully, maybe helper air bags. But I'm going to suspect you'll want a 3500+ truck, long bed, duallie in the long run to tow this 5th wheel. It will make the drive a lot more relaxing, which is a major part of having fun. Unless your definition of fun is code brown moments when trucks pass you while hitting bumps
.. In which case I respect your definition of fun, but prefer to watch it on youtube.
Anytime you exceed GVWR you do open the (remote) possibility of getting a ticket. Lets face it, cops are only worried about REAL safety issues like going 5 over the speed limit and parking in a no parking zone. They don't care if your texting while driving an overloaded truck, so long as your not speeding or drunk or both haha ;-) They really don't want to follow you to a cat scale for 45 minutes to give out one ticket, when their performance appraisal depends on how many tickets they issue.
So just keep it below 80 while your texting and having your roadie and you'll be fine.