You'll be surprised how fast the weight goes up as you load even just the essentials. Pots, pans, plates, utensils, lawn chairs, hoses, electric cords, on and on, not to mention people and pets.
At the least you want to know the empty weight "wet", with full fuel and water tanks. Compare that to the gross vehicle weight, which should be on a sticker on the wall next to the driver's seat. Do NOT use the shipping weight for any calculations.
I'd think a minimum of a 4,000 lb usefull load (the difference between you wet weight and the CCC - cargo carry capacity), but even that can be exceeded if you're not careful. Things get a bit more complicated though when you stop to consider where that load is (axle weight). Many coaches seem to have a problem with, especially, front axle weight capacity being exceeded. Some have reported here that their 40' coaches have less than a 2,000 lb CCC after fuel and water are added to the equation (manufacturers have not always considered the practical use of their product, they're only interested on getting it out the door).
Shop carefully and you won't have to worry about overloading your rig. Ours, as example, with full fuel and water (100 gallons) still has almost a 9,000 lb CCC.
No doubt any of the toads meeting the parameters you've suggested will fall under the tow capacity of any 40 or 43' rig, but you do still have to be cognizant of the gross combined weight rating - the weight of the loaded rig plus the weight of the toad. That gross number should be on that same tag.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
Wrangler Unlimited toad