More downsides to carting a lot of batteries about
1) Recharging them. You'll need a 12V car charger which will take from 4 hours to overnight to do one battery. I've used a 3 way adapter to rewire three SLAs into one parallel 12V battery instead of the 36V serial battery needed to ride, and a smaller car charger took over 24 hours to charge them all at once.
You won't be able to use the bike's Lithium charger to charge the SLA batteries, because the chemistry is wrong. So it means packing along two chargers if you intend doing an out-and-back overnight trip.
2) SLA batteries are reasonably robust now, but many still contain liquid acid. "technically speaking" you should have a diamond-shaped hazardous cargo sign displayed. There is a risk of cracking, or I managed to wear partially through the plastic casing on one battery, so there's always a risk of an acid spill/release.
3) Wiring - your ebike will expect to receive 48V DC at roughly 6-7 Amps current into the system, assuming a 300W motor. The SLA batteries on the back will have to have a strong but thick power cable to get to the battery fitment. Remember thin wires means loss, so you'll expect something at least as thick as cheaper jumpstart leads.
It will have to follow the bike's draw-bar for support, but still have enough flex to turn corners. Likely you want to disconnect the trailer at some point for charging, so a plug in the line will be good too.
Perhaps you're approaching the ebike the wrong way. The electric motor is an assistant to your inputs, and its not the other way around. For your long rides, you aim to use the assist judiciously and only when its needed like climbs, not all the time.
Should the battery run flat, well you still have your legs. Downside is your flat ebike is now just a really heavy normal bike.