Until earlier this year I had a Chateau Class C, had the antenna stuck up against the ceiling above the overhead bunk. Fiberglass cap on the unit, but I had dropouts about 50% of the time while in motion. Parked, not so bad. I tried forward on top the dash through the windshield like others have suggested, it was worse than on the ceiling. This was using an XM receiver.
I had come to the conclusion I was going to have to install an antenna on the roof, was strongly considering a high-gain marine unit since boats usually don't have good ground planes, just as most RV's don't. But then we decided to trade the regular Class C for our current Super C. I installed an external mirror-mount "trucker" antenna on the current unit, it works great at all times. Very few dropouts, no worse than my factory-equipped Ford Explorer. Both those radios are on the Sirius satellites.
I realize many units don't have the big mirrors a Seneca does, but getting the antenna up high is probably what will be necessary. I also think geographic location is important, and possibly even which service you have in a given area. XM works better in some areas and Sirius in others. Some radios can reportedly tap into both networks. Others are limited to one service or another. Even though they are one company, they still have 2 separate satellite networks.