All depends on how handy you are with this set up type stuff.
If your comfortable with it, go with the same one you have at the house, this way you don't need to make any changes in the recvr.
You didn't say if the recvr is coming from the house to the RV and back.
IF it's only staying in the RV and HD will NOT be something you'll want to get into now or in the future, then I would say just use a simple single LNB dish like CAVE mentioned, you'll want to run the Repeat Sat Set Up when setting it up in the RV so you can set it for the correct dish type.
If your thinking of going HD at anytime soon, then I would recommend getting the Slimline disah right from the get go, so you don't need to get a different dish when you make the change.
Btw, I'm guessing you already have a Tripod ?
Fwiw, the old single (or triple LNB) dish uses a 1 5/8 pipe (mast), while the Slimline dishes are abit bigger and require a 2" pipe., this is important to know when buying the tripod.
Also get yourself a compass and a level.