A company named Wilson Electronics makes cell signal boosters that work in your car, truck or RV. They sell a model for $199 and a better one, designed specifically for the RV, for $599.
I bought the cheaper one 3 years ago and tested it in REMOTE areas of eastern Tennessee near the Smoky Mountain National Park. Areas where the signal read "1X" and zero bars. You turn on the Wilson, drop your cell phone into the bracket and your signal gets multiplied 4-5 times stronger.
Now, to be clear, if there is NO SIGNAL, it will not work. You have to have some kind of cell signal, and it will amplify that. Their website includes a product selector wizard to guide you.
I was told about this product by the guy I bought my property from, as it is 3 miles from the National park and the mountains are 2000-6000 feet high. Cell signals are present but weak, so you can get text and email, but not make outbound calls, UNLESS YOU USE THE WILSON BOOSTER.
Anyway, I was quite pleased with it. It should do exactly what you want.
weBoost 470107 Drive 4G-S Cradle Signal Booster Kit