My signature combo... Trailer weighs approx 6000 before water and bike.
Having a front bed is great because you can stop overnight and not need to unload the bike to use the trailer. I do wish I had a 24' instead of the 21' simply because my street bike (VTX1800T, 8'2" overall) protrudes into the living area a little bit too much. On my trailer, it also eliminates the ability to use the built in table/couches located at the back of the trailer.
When I was using it as a base camp for an extended work arrangement away from home, I parked my bike in it as a garage and just use a small chair and folding table. I kept the bike tucked up tight against one wall. But this was only me being in the trailer, it would have been tight for 2 people.
Running 65mph avg, I get 12-13mpg, sometimes in the 14's. With my truck, I have plenty of capacity in both weight and power. I do not feel my 700+lb tongue weight. The trailer rarely pulls me out of 4th on the steep interstate hills. I do not plan routes or worry about what roads to take.
We've been from LA to Glacier, interstate and 2-lane. I've never had an issue with hills, temperatures, or feeling underpowered.
With the available vehicles, I would go either a newer gas or maybe an older diesel; pricing would be similar. With my current trailer and something terrible happening to my truck, I'd be looking really closely at a Ford V10, Ram 6.4L, or maybe even a 6.0L or 8.1 GM depending on what I found available to me. I'm really attracted to the Ram Ecodiesel, but would be close to ratings after we packed up for a family of 4. Ford Ecoboost is another possibility.