Since you said fulltime I'll assume a DP. First diesel will last... not go bad for years unlike the current gas that will hardly run in a 2cycle engine if 6 months old. The nice thing about diesel engines, is they can sit for years and put a new battery in and and they start right up so every 2-3 months is BS. If anything, I'd worry not running a gas MH/generator every 3 months... not a diesel because of the sorry gas today.
Since you will pull a toad behind your DP and will put close equal amount of miles on both (NO, I do not mean those behind the MH :S) your overall MPG could be as high as 20 MPG. 7 for the DP and 33 for the toad = 20 average MPG which is better than any 5er combo.
Having had a 5er, there is no way I'd go back and we snowbird for 3 months (yes it's not started) and usually go out west for 3 more. Unfortunately those summer plans went south and the DP has been sitting since April and will probably not get started until the end of December... 9 months so I practice what I suggest.
Especially for FT, buy more MH than you think you need... ONE TIME!