You have a similar charge controller to what I have in our 89 Sunline...and in a storage shed I have. What I would suggest is just ignoring the load output on the charge controller. How is your trailer wired for 12v? In our 89, the 12v converter/charger unit has a battery connection, and something like 4 fused circuits going out. One of those circuits is tied in with the battery to filter the power. I pulled the battery filtered circuit wire out of the charger and added in a small 12v fuse panel. At the fuse panel, the feed in comes from the converter (going in where I removed the one just mentioned) via a 10 gauge wire with a 30 amp fuse in the converter for that circuit (upgraded from the 15 amp since I replaced it with #10 wire). At the new panel I connected the old feed to a 15a fuse (matching the original), and the battery feed from the charge controller to a 10a fuse. Could have used 15a, maybe even 20 since I was running 12 gauge wire to the solar panels...but I knew I wouldn't get over (or close to) 10a out of the panels...two 50w in my case. From there, connect your solar panel into the solar panel input on the charge controller, and you're in business. The pump and all the rest of your internal 12v circuits should continue to work as they always have.
Hope that makes sense! If not, ask and I'll try to clarify...
Please be careful with properly fusing things. I think many people get confused as to the purpose and sizing of a fuse. The point is to protect the wire. If you get too much current flowing through a wire, it will get hot and eventually catch fire. The fuse is there to keep it from carrying too much current. 14 gauge wire gets a 15a fuse, 12 gauge 20a, and 10 gauge 30a.
Wesley
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