Bill what he’s describing is that you would take the HDMI signal that goes into the back of the TV in his RV and split it. That means you watch whatever channel he selected over there.
There’s probably no HDMI port or cable into the RV, so it would have to come through a window on both. Not ideal.
To do a true two receiver Dish setup is dependent on exactly what kind of dish he has (self aimed, motorized winguard or similar, or their tailgater model). All need different main and second receivers last I looked into it.
If Dish has a wireless second receiver that works with his main one that wouldn’t be too bad. You’d be close enough for that to work and he would just hand it and it’s remote to you and you’d hook it to your TV in your RV.
If either RV has an outdoor TV like some of the fancy ones do, that getting the HDMI outside might be easier. There might be a feed cable to that TV and snaking a cable out of there as long as it wasn’t slammed in the outside hatch door when closed would be easier. If his indoor and outdoor TVs alee already have separate receivers for each from the dish and he has an RF remote for the outside, not an infrared one, you’d have channel control of the second receiver he’s sharing.
There’s also ways to split the signal coming off the dish but it requires two compatible dish receivers probably on the same account.
So however you slice it you’ll need to know his exact setup before you’ll know what you guys can do to share it. Whether from the dish itself with two receivers, or sending his digital signal of the channel he’s watching over HDMI over to your TV.
Oh yeah there’s also wireless HDMI extenders. Again splitting his picture and wirelessly sending it over to your TV.
Bunch of ways to do it, all different prices, some with a way for you to control channels, some without.