As a Direct subscriber the problem you'll have is with the automatic dome based antennas can not access the signal for HD channels from Direct. As such you're looking at the two ends of the cost spectrum if you want HD. Either a manual aimed residential based antenna mounted on a tripod as jerseyjim described or something such as a Winegard Trav'ler. Anything in between will work just no HD channels.
Roof mounted antennas work well but you could run into problem with the line of sight between the antenna and the satellite blocked by trees, mountains, buildings,,,. Last owner of my coach install a roof antenna but I also have a carry out with 100' of cable which gives me flexibility to find a clear view if the roof antenna is blocked. I use the carry out on average 3 out of 4 trips due to blockage. I only have a single receiver so the signal feed was simple using the existing coax. Since your intended installation is two receivers you can run into problems with a some dome based antennas in that the antenna can only receive a signal from one satellite at a time. If you are using two receivers there can be problems if the channels for each receiver is on different satellites. With either a residential antenna or the Trav'ler that won't be a problem.
The amplifier for OTA channels is in the antenna. Power for the amp is injected on the coax going to the antenna.