2-way sat modems are very tricky to set up the dish. You can't just point them with a compass and azimuth guess like you do with DBS...you have to get feedback about how well the satellite is receiving your uplink. And if you do get it pointed correctly, every time you walk around the RV you'll move the dish a little bit and lose the uplink. Also, the "flat" dishes you see on top of escalades that work in motion are receive only. You cannot use a 2-way sat modem while in motion, period.
Inmarsat has some neat solutions to handle the constant motion of ships, which are always shifting slightly and rocking in the water... It involves a fancy dish/panel that is aimed at the correct satellite, and as the ship moves, turns, or otherwise changes its position, this little panel will go up and down and turn to remain aimed at the satellite. I believe you can get ISDN speeds and/or 2 concurrent phone calls with it.
Another solution I've seen is what the Red Cross uses for internet on their Emergency Communications Response Vehicle ... they have a dish that collapses down when moving, and when stationary it can be up and running within minutes I believe.
Link about the ECRV: