Many of the Monacos single axel are wandering beasts. I know, I have one that has been tamed.
Be sure to test drive at highway speeds. Nice RV, it was just a beast to drive. Some earlier models had trailing arm issues.
You should be able to get an excellent RV for the budget. If you keep the price down you will have plenty to trick the new unit out to your likings.
The 400 ISL and larger are excellent choices in engines. Cummins that is. Not familiar with others.
Ditto on the IRV@.com site. There are mfgr specific areas so you can really drill down and get responses from owners of the rv you like when you have it narrowed down.