"I use to advocate carrying a spare in my 2005 Monaco Diplomat. On that coach, it used a short 22.5 tire and it fit in my pass thru bay, unmounted. I could stuff a lot inside the tire and didn't lose much storage. Fast forward to my current RV and the 315's won't fit in a bay.
So......a lot of people say to just carry a spare, including a trailer owner with a rear mounted spare. It's nice to say and actually nice to carry a spare if you have room, but if you don't have the space, you need another plan. That plan is to have a roadside service plan and be willing to accept an odd sized or used tire to get you back on the road. Wait until you're in a major town on a weekday to resolve the issue with a new or correct size."
While using an "odd" size on the front might get you moving (slower) mismatched duals will mean the larger OD tire is carrying more than design load.
I would limit the speed to 50 or less and be prepared to scrap BOTH tires as the internal structural damage is impossible to "inspect" for.
If you have two different size tires on your RV and can only carry one I would suggest the rear. Hopefully, you have confirmed your actual load on each tire position and have some margin available on the fronts. If you are running at 100% front capacity (bad idea) then the use of a smaller "rear" tire as a temporary front means you really need to limit both speed and distance you operate in that situation.
Overload does not always mean tire belts will overheat and a warning is received from your TPMS. The steel sidewall ply can fatigue from over deflection and you will have no warning and not know it till the sidewall fails catastrophically.
All the above brings up the question of why did you have a tire failure in the first place?. Have you read THIS post.
A properly programmed TPMS should provide an early warning as soon as you lose 15% from your "set" pressure (slow leak) or 3 psi from your hot pressure within a couple of minutes (fast leak). If you are also having a tire professional do a close and thorough inspection every year starting at 5 years for Class-A and Class-C or at 3 years for any trailer, it is unlikely for you to develop a belt separation without some advance warning.Taking precautions is always better than trying to "fix" the tire after you have a flat.