LOL we had the reverse problem on LI...we have Verizon service but the only signal you can get on the beach is AT&T. I got a prepaid ("burner") phone at the AT&T store and set up call forwarding from my Verizon number to the AT&T phone. Not sure if you can forward texts from your phone to the burner. Verizon has prepaid phones too.
Option #2 is get a Verizon JetPack hot spot to connect you to the web and enable wifi calling on your AT&T phone, this has also worked well for us. Having your AT&T phone connected to wifi via hotspot also gives you chat, text and whatever else you need and is probably the best solution if you're hanging around the camper.
The prepaid burner phones are OK, but the minutes expire. Invariably I would wind up losing airtime because I wanted to be sure I had enough for the week, but the leftovers would expire before the next season's trip.
Another option is a signal booster on a long pole. I had a signal booster from
Powerful Signal and a trucker's antenna that I mounted on a 30' telescoping flag pole. Bungess the pole to the RV ladder and the extra height let it "see" the signal I was looking for, then the booster would bring it in.