We have DirectTV in our motor home. When we had it installed, we used the cable connection already in the motor home for the signal from the satellite dish. The only problem with doing this is the existing cable will only carry the signal from one LNB, which means if you want DVR (TiVo) you won't be able to record one program while you watch another.
The biggest problem we had with the installation was finding an installer with a tripod. If you can find a tripod from DirectTV, they are the type used to mount the dish on the roof of a permanent building. They are very heavy and not very convenient to adjust. They are designed to be locked down in one position. We had DirectTV installed in the motor home before we went full-time while still at home, so when we couldn't find a tripod, we just had the installer mount the dish on a fence post. We bought our own tripod later
If you already have a roof-mounted dish on your RV you are probably are OK. If you want a roof-mounted dish, but don't have one, then you will probably need to go to an RV dealer or Camping World to have one installed. I don't think any regular DirectTV installer will do this.
If you want a tripod, then my recommendation is buy it ahead of time. There are many good ones available, including some from Camping World or one called
Bullseye Mount.