If the polarity of the SAE connector on your trickle charger matches that of the "port" it should work just fine. The port should be wired directly to the batteries which should be wired in parallel.
One thing to note however. If you have two batteries wired in parallel, that means that you have twice the self discharge current of a single battery. Add that to any parasitic draws like LPG leak detector, etc, and a 9 watt trickle charger may be struggling to keep up.
If you already have the trickle charger, give it a shot. If it doesn't keep up look at a larger size, perhaps a "Smart" charger like those from Battery Tender, NOCO, or CTEK. "Smart Chargers" will sense the state of charge on the battery(s) and when fully charged will drop to a float charge. If the self discharge and parasitic loads draw the battery down any, the smart chargers kick on a higher charge rate until the battery is back to 100% SOC again.
"A wise man can change his mind. A fool never will."
"Sometimes you're the dog, sometimes you're the tree"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan