Toy haulers generally have more pin weight, about 25 percent, compared to a normal 5th wheel being closer to 20 percent. This means a possible pin wt of 3K, if looking at a Toy hauler with 12K GVWR. You will need to weigh your truck, loaded ready to go, full fuel, weigh rear axle to see what is left, before going over RAWR/tire max rating.
RVs normally do not have to stop at DOT scales. As to being a target with triple axle, if you look like you are way over weight, look unsafe to be towing whatever rig, you could get pulled over by any LEO.
GMs have a longer axle center, to back of cab, than others. You may not need a sliding hitch. Many do not, depending on front of trailer design, and if pin box extends to front.
Jerry