Well... You don't state what type of van, engine, etc, or what kind of trailer, but, here goes
The 80% rule is one of those "myths" that doesn't go away. In the "good old days", when hitches were cruder or just plain hazardous (clamp on bumper styles
) that 80% figure was to give people a false sense of safety. I've never seen any scientific basis for an 80% rule other than a "feel good" psychological sense of well being. A manufacturer builds a vehicle, then tests it to come up with a "factory backed, warranty supported" towing limit. If the factory says, and warranties, that you can tow to 3500 lbs, then 3500 is the upper limit. Anything under that doesn't mean that it is inherently safer or more reliable. Vehicle wear and tear, of course, goes down with declining tow weight, but, even that is deceiving. Sail area also has to be taken into account and manufacturers usually give a maximum specification for that along with their towing limits. A 3500 lb trailer with a full width front will be much harder to tow than the utility trailer that weighs 3500 lbs. Most towing limits, btw, are done with utility type trailers and the actual towing capacity for any given vehicle usually decreases as the sail area increases.
With a 3500 weight limit, I'm going to guess, at most a small V6 engine with an automatic...4 spd? Mini-van? At any rate, that 3500 lbs is aimed, primarily, at a utility trailer or, possibly, a small pup-up type TT. More than likely you have got a Class III hitch which severely limits your tongue weight, which, btw, is not added back to your trailer weight, but, must be added to your TV weight, along with gear, options and passengers. You don't mention how many folks, but, a real concern is going to be overloading your "van" if you aim to carry what most of us pack in the travel trailer inside your van. You'll also need to add the weight of the hitch/anti-sway assembly to your vehicle load and you'll also need to add that figure to the tongue weight of the trailer to get a "true tongue weight" and then look to see that your Class III hitch can support it without separating from your van...
You also don't mention what type of trailer, but, since this is a CruiserRv (now) forum, I'll have to assume you are talking about a full size travel trailer and not a pop-up. Beware the published empty weight figures and the published tongue weight figures. Those figures are based on an empty and un-optioned travel trailer. The true tongue weight will go up as you add the 12v battery (required for the break away switch) and the weight of the LPG tank and gas. Any "options", such as awning, microwave, television, etc. will add to the published empty weight. You may find that your "advertised" empty weight is not 2525 lbs, but, closer to 2900 lbs and your tongue weight for a safe tow is going to need to be 12-13% of that or roughly 340 to 380 lbs...
More information would help, but, with what you've given in your post, I don't see how you are going to manage safely and without severe vehicle wear and tear or outright damage from overloading the vehicles frame, suspension and powertrain.