I'm betting the 12000lb number is the GCVWR (combined truck and trailer max weight). That is only one limit:
- Max payload of the truck? (hitch weight is part of this and likely north of 900lb for a 7000lb trailer)
- Max hitch weight which it appears to be over (you might get higher with a weight distributing hitch).
- Tow rating of truck: basically pulling power
- Rear Axle Rating
- Maximum Frontal area: An RV has a lot more drag than a low utility trailer of the same weight.
Pulling the stats for your truck would be the first step but it looks like you are at or above a few of the limits. This assumes you carry little to no other cargo in the truck.