The dynamics of trailer towing are pretty complicated and the limits in the manual are as much to protect Land Rover as they are you. Having greater tongue weight is generally better until you over load the rear axle on your LR3. Having negative tongue weight from, say, not pulling a boat all the up onto a trailer will result in a very dangerous situation. The best way to avoid problems is to load your camper or boat and the rear cargo area of your vehicle as you normally would for a trip and have the the rear axle load weighed. Most people wont have a facility to do this. For a small trailer, a simple bathroom scale under the tongue dolly will give you a reading up to 250 pounds. For anything other than a small utility trailer, if you can lift it, it's not enough tongue weight.
Other things to consider are to make sure that the trailer is level. A trailer that isn't level will almost always result in unstable swaying. Keep the weight in the trailer as low as possible and as close to being over the axle(s) as possible. An extreme example of this is keeping the sails of a sailboat in a trailer box instead of in the boat. A 30 foot fixed keel boat might weigh only 7500 pounds but could have 1000 pounds of gear stored eight feet above the axle centers. Weight up high like that could result in a dramatic change in tongue weight during braking and acceleration. So much so that during hard braking the tongue will push down on the rear and unweight the front axle dramatically reducing braking ability.
Consider what additional gear you might carry in your travel. Most units have large storage areas under rear beds; tools, cast iron dutch ovens, climbing gear, tent canopies might be more safely carried in the center on the floor rather than behind the axles.
Another area of consideration is the aerodynamics of the trailer. Here in S. Florida, sport fishing boats with multiple outboards and a fixed "T" top over the helm are very popular. In this case, a lot of weight is carried behind the axles and there is a wing over the center. At interstate speed, the "T" top effectively shifts the weight off the tongue and what my have been a flat trailer at rest now might have very little tongue weight.
In the end, your LR3 with its self leveling suspension is one of the best towing rigs ever built for loads up to 7500 pounds. Maintain adequate tongue weight, keep the load low and centered, and make sure the trailer is level and other than reduced acceleration you likely wont even notice that your towing. You shouldn't have any problems with your 5000 pound travel trailer.
