Is it possible to build something to extend your range.. Two answers both yes
First if you are camp hosting does the park offer WI-Fi. just your RV is too far away to log in solid?
T-mobile offers a "Mini-cell" this is a device that plugs into the ROUTER on a LAN port and basically becomes a T-Mobile tower. Not sure the range but it might do the trick, also I BELIEVE (again not sure) you get UNLIMITED everything when using the mini-cell This is the exact opposite of a hot spot. Where as a Hot Spot gives you Interenet via cell. this gives you cell via Internet.. your Hot spot would then convert back.
Second.. There are several devices sold to extend Cell range on RV's and such We-Boost and one other (max Signal?????) come to mind, the other is the better one range wise.
These devices use an extenal antenna and ... generally.. and internal antenna with a box of electronics between them.
Use a DIRECTIONAL high gain antenna atop a good tall tower (Say 40' flag pole) because HEIGHT IS MIGHT.. Note T-Mobile's engineers may be able to advise on how tall a pole.. Now some of them are not that good with data (They may force you to a slowere frequency band) but I'm not currently using them so I can not be sure.
I can also extend your WI-Fi Range for the park router if the park has enough bandwidth, That I am very good at. Two or 3 different methods some are downright impressive. (NOTE: I do not give the actual instructions. only links to them)
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times