Going off the currents you mention, a massive alternator won't help. Its the cabling that is the problem the cable size you need will be in the rough size
of 50mm 2 ,
Use LEDs wherever possible and that will keep the current draw to the barest minimum.
Yes 50 mm work it out, don't forget you will have a long length of cable from the front of the vehicle all the way down to the trailer and you will have losses
in the plug and socket, so the combination of the length and the size will determine the volt drop
admitting using LED will be better but going off your original values you need a very thick length of cable from front to back
You can try Mike at Robson and Francis. The number is on here.Wheres best for upgraded alternators, just for a price?
Thats the one..You can do your voltage drop calculations at the link below once you know all your amps. 50mm does seem like it would be over kill.
http://www.12voltplanet.co.uk/cable-sizing-selection.html