Electricity to my Ice-cream van from 12v alternator by using mppt charger

You cannot do this at a low cost. 7000 watts on a 12v system is extremely difficult, very complex and super costly to do. You would have to divide it into multiple systems. While not technically impossible, this is darn close to it.

Example: Your alternator is likely rated for 1400-2000 watts peak. It was not designed to do that continuously, so it would burn out prematurely if forced to do so.

Note how far apart 7000 and 2000 are!

Are you sure that your load is 7000 watts?

If your van is a 12 volt system already, your alternator will put out about 14 volt of regulated power to charge the 12 volt starter battery of your van. No charge controller is needed to charge your starter battery. If you have extra batteries, the alternator can supply power to the extra batteries also and there are different ways to do it.
Do you know how many amps your alternator can put out?
A alternator for cars can range from 40 amps to about 60 amps and 80 amps to 120 amps for trucks and vans. And there is always a special alternator that may have different voltages and amps.
A 80 amp alternator will give you 1120 watts an hour at 14 volts, but the vehicle itself  will use/need  about 20 amps to 30 amps so it can operate and run,  so your available power  that you can use from the alternator will be even less for other things.

There are pricey alternators on the market that put out a lot of amps and may even fit your van without major modifications. Those alternators may come in different voltages and there are some where the alternator output amps and volts can be adjusted. BUT you got to have a ICE (internal combustion engine) big enough  that can supply the power needed.