I’ve looked at them as well as it seems a great alternative to maintain power, especially in combination with upgraded solar on the boat.
We get about two days at anchor in our R27 before we start to bump against 50% remaining without strong solar contribution. Add improved solar and we probably get 3 days. Add a 100AH batt pack and a decent portable solar for it, recharge the boats usable 100AH every three days via a shore power connector and recharge the battery bank for the next three days via solar, boom - endless power!
Seems that these things are game changers and awesome ways for us without lithium on the boat to bring lithium into the boat!!!
I’m tempted to buy an Amazon deal too now but the need for more power won’t come until spring, and they might be even cheaper then.