We have been full-timing for 10 years and decided on our Bighorn 2 years ago. This is our 2nd rig since starting this lifestyle. We used our previous rig for week-ends and vacations for 1 year, before going full-time. It served us well, and we could have gotten several more years out of it, but couldn't pass up the deal we got for this one. Whether the Bighorn will last as long remains to be determined. Since we've slowed down some now, spending more time in one spot, we hope that it will do just that. We don't feel that this unit is built as well as the previous one, but there are trade-offs on everything, and we knew that going into this unit. So far we've had the problems that most have written about on this forum, but we had problems with the other one as well, just part of what you can expect when you decide to live this life. Good luck in your quest. We would not give up this lifestyle unless somehow forced to by circumstances beyond our control.
To answer your other question, yes we planned on having to replace our rig at some point, as well as the truck. If luck stays with us, we plan to replace this one as well (we gotta wear it out). We have friends, also full-timers, who have replaced their diesel pusher about the same time we replaced our 5er' and same length of time in use. So it doesn't make much difference what you get into, these things don't last forever, but will go a long time with some care. By the way, we bought our 2nd unit after stopping at a dealership when we left a tire shop having set an appointment to put on a 3rd set of tires on the 5'er. Probably would have been cheaper to replace the tires!
Just to add to your thinking process, we are on our 2nd truck, also. The first one was a gas (big mistake), but it served us well for 6 years and 150,000 miles. DW said, no more mountains unless we got a new truck. Finally got my diesel, wow, should have started with one. This truck should be the only one we'll need from now 'til we stop traveling, but...
Bill