Looking at the two tent bed supports in this google photo doesn't look like it would be too big a deal to modify as necessary. They look to come almost straight down from the tent bed area to the tongue area.
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They may not need to be modified at all after you move the PROPANE TANKS up to that area.
The tanks will also have to modified with adding a tongue tray that may extend out on both sides just a little as well as adding a new arrangement for the fuel lines.
Nothing comes easy when you start modifying things.
The weight of the batteries almost wants you to be as far back as you can get on the trailer tongue frame. Even up against the front of the trailer the 48-inch metal box is going to extend out over both sides of the trailer tongue. My planes is to use automotive round rubber mounts between the bottom of the tool box and the top of the tongue frame. I also will be using PVC Electrical boxes and PVD FLEX tubing (LOWES) to route all of the required battery cables and control lines in getting back inside my trailer.
The extended LIP area of the 48-inch tool box is perfect for installing all of your battery cable routing to the battery terminals and also a great area to mount the required four position BLUE SEA Switches and terminal blocks. I really liked the way that LOWES SINK DRAINs was used to provide battery vents next to each installed battery. Great idea from 'gilldawg79'
You really will need to come up with a lockable metal latch battery box to help prevent battery thief. It is becoming a real problem around these parts keeping the Propane Tanks and Batteries out of the hands of thieves. These items get used in making drugs I guess...
It is much harder for the thieves to bust into a metal lock latch verses just cutting off pad locks with their portable grinder tools. Along this line of thinking I will also be using the ROUND STOVE BOLT heads on the bottom of the trailer tongue with the bolts and washers being insides the bottom of the metal tool box. You have to get the tool box opened to steal it...
I'm sure you can come up with something - Your project not mine haha... Just passing along food for thought...
Roy Ken