I used to travel 500 Km to my native village with a fully loaded vehicle along with 5 person in my WagonR. I also used to cruise with speed in between 80 to 100 Km/hr but the vehicle braking decrease very much in respect to only travelling with 5 person with little baggage.
Might need better tires, may need bigger brakes. Make sure you're using appropriate tires for the load and conditions. Consider trying better brake pads or possibly upgrading to a big brake kit. Perhaps better brake cooling (such as cool air ducts) would help.
First you're going to have to identify the weakest component in the list so that you can upgrade it.
Tires are going to be the biggest change you can implement. The widest, stickiest tires you can get for your conditions.
Typically, any car can lock its tires up under normal conditions (average load, speeds). If it can do this, the braking system is more than adequate, and the tires are the first thing you should change. Now, when you start upgrading tires, the braking system typically starts to fall behind. Then you can worry about upgrading other braking components.
If you have a heavy load, I'm betting your getting pretty good grip out of your tires, so getting better discs, calipers, etc. would be the next step. Unfortunately, this is an upgrade that's more aimed at racing enthusiasts, so finding parts suitable for street use might be difficult.
There are a few things that you can do to gain better braking ability.
Upgrade your brake lines
Vehicles from factory usually come with stock rubber brake lines. Upgrading to stainless steel brake lines will lead to less 'gummy' feel when you step on the brakes. This also has the added effect of allowing more even pressure to be put on the brakes. Read this article and this wikipedia entry on braided stainless steel brake lines for more info.
Upgrade your brake pads
Upgrading from regular brake pads to track-style brake pads will greatly increase your braking performance. Track-style or racing brake pads usually tend to be louder under braking and tend to create a lot more brake dust, but provide amazing performance in comparison with stock brake pads.
Upgrade your brake fluid
Upgrading your brake fluid from regular brake fluid to racing or track-specific brake fluid will again increase your braking performance. I use Super Blue Racing Fluid in my car.
Upgrade your Rotors
Upgrade your rotors to slotted or drilled or slotted-and-drilled for increased performance. Also, the bigger the rotors, the bigger area the pads have to bite and the more easily heat is dissipated.
Upgrade your tires
Upgrade your tires from regular street tires to high-performance summer tires or high-performance all-season tires. Tires like Michelin Pilot Sport PS2, Ling Long L688, Nitto Invo, Yokohama S.Drive, Bridgestone Potenza RE760 Sport, Kumho Ecsta XS, etc are good tires with great bite.
Upgrade your driving style
Remember that with a higher load, your braking distance increases greatly.
If this is a vehicle that you just bought it is possible that the brakes just aren't as good as your previous vehicle. From the information I could get online, this vehicle is equipped with drum type rear brakes. The problem may be that they are not adjusted correctly. Although the front discs do the majority of the braking ,the rear brakes must be adjusted correctly to achieve maximum braking. To an extent the rears brakes are self adjusting. There are a variety of self adjuster types from different manufacturers. Some require that you drive in reverse and apply quick pumps to the brake pedal, other require setting the park brake several times. The best self adjusters are marginal at best. The best way is to periodically have them adjusted as part of routine service. If after verifying that the brakes are working correctly you are still dissatisfied with their performance you may want to look at upgrades. The original pads were installed as a compromise of cost, noise, durability and stopping power. To stop faster you will most likely give up one or more of these characteristics. They may not last as long, they may wear rotors faster or make more noise. They certainly will cost more. Vented and drilled rotors will give better brake performance but at an increase in cost. An alternative would be to use engine braking.
Do a thorough inspection of the entire brake system, flush the fluid and replace any obviously out of spec parts. You say that the stock configuration worked fine for some time. That indicates to me that you need to restore it to the stock configuration and functionality that worked fine in the past. Racing pads, cooling ducts, sticky tires, etc. are solutions for different problems.
When brakes go from OK to Not OK, I suspect brake lines unless there are other obvious symptoms. As brake hoses age, they wear and crack which allows flex when you step on the pedal. Energy is spent inflating the hoses, not squeezing the calipers. Inspect the brake hoses (the flexible rubber ones between the brakes and chassis). If they are not new or newish, replace them with factory parts at least, stainless steel hoses would be better, in my opinion.