Backing plates are usually 1/4" thick aluminum. I'm using 3/8" stainless carriage bolts with my backing plates which are 5x6", still using the threaded eyes that FWC uses. FWC's turnbuckles are identical to those I've seen at Home Depot. I am using those now but plan to replace with stainless steel safety wired ones.
On electrical FWC bases their hookup only on the 30 amp system they have installed in the camper, ie it's based on the load from the appliances. The plugs are ones from marine trolling motor hookups, they have used at least two brands over the years, The Marinco is the older one, they seem to be using Attwood now. But if you are trying to get better charging of the House Battery Bank in the camper then you can feed more amps off the alternator of the truck using other choices and designs.
My Ranger has a 170 amp alternator, just the standard size for the 3 liter 4x4 Ranger in 2000. Because of wiring sizes and so on in the ranger I don't figure the alternator can feed anything like that amps to the ranger's own battery let alone out through the camper connection to the house battery. So I'm using 60 amp circuit breakers feeding 8 gauge wire out to the camper, breakers must be at both ends as there are batteries at both ends and the system starts direct from the heavy battery connections. The hookup connectors FWC uses are marine connectors designed for hooking up trolling motors. But only designed to attach 10 gauge wire. Various brands of those connectors are available for various gauge wire. So the connectors I'm using for the hookup are another company, Sierra WH10530 on the 8 gauge cable from the camper side, WH10520 is the truck side plug. That's all 8 gauge throughout and less than 15' of run, which with the distance involved the wire should handle 70 amps safely, thus the 60 amp breakers primarily to protect the wire. Isolation in my system is provided by a BEP marine 710-125A Single Sense Voltage Sensitive Relay which can handle 125 amps, and automatically hooks the camper feed up when the Ranger's voltage reaches charging level. (it would also be possible to jump past the VSR to help start the Ranger if needed, though 8 guage would limit that to brief assist only) My House battery bank is two Northstar NSB-100 FT batteries, totaling 200 amp hours, capable of handling far greater charge rates than I can supply, so the bigger the rate the less charging time involved. I do have the same 30 amp converter from 110 that can also charge the battery bank, but having a greater charge current than that provides is an advantage. And I'm putting in monitoring which will fully keep track of charge levels directly. All my parts for the setup came off Ebay
I'm still working on the camper, so I've not yet put my meters on the feed to the camper to find out just how much of a charge rate I'll really have. (my original DC feed for trailers and 12volt use was 10 gauge and a 30 amp fuse, not good enough, though I'm still considering keeping it in supplying only the trailer hookup where it was plenty)
My solar setup is a dual morningstar controller, so it can charge the ranger's battery as well as the house batteries. It gives priority to the house batteries. But it will be a fairly small source, probably never put more than 200 watts of panels on, probably half that.
The thing to do in your setup is calculate the possible loads and charge sources and then decide on connectors. Feeding the camper's appliances may be the smaller of the currents to contend with. Solid state isolators will really limit your charging ability due to voltage loss. I've used VSRs on my boats and won't go back to the diode setups. And make sure and put circuit breakers or fuses on both the house and truck battery end.
Someone talked of using the AC plug systems used in campgrounds. I'd not do that unless one day you want someone in error to hook up 110 AC to your 12 volt system. Also note the ratings on 110 volt AC plugs don't relate to the ratings on 12 volt DC plugs. Stick with plugs made for and rated for 12 volt DC use.
