I think the concept of golf cart batteries and an invertor in the bed of a truck is a good concept. Lewster makes a good point about ventilation, so this needs to be considered.
I think that I would have one battery in the trailer to provide 12 volts when towing and whenever I needed 12 volt
power when boondocking and the truck was not there. The golf cart batteries would typically be used in conjuction with the invertor to provide 120v ac power to the trailer for 120v stuff and also to the convertor for 12 volt
stuff. Recharging of the batteries in the trailer (1) and the truck bed (2, 4 or more) could be by solar or a small 1000 watt invertor type generator.
I considered using #4 welding cable but ended up buying some high quality 24 ft long jumper cables with #4 wire. The cost was much less than welding wire and they are just as flexible. When I am done I think I will still have some 10ft long jumper cables that I can use in an emergency.
I am using 4 6v golf cart batteries with a 1250 watt invertor. Some may think that I need #2 wire. I am using #4 wire. If the wire gets too hot when using the invertor I will just run an extra line of #4 wire from two of the batteries to the invertor (less than a 2 ft run).