What do you suggest to prevent degradation of RNA isolated from tissue with high amounts of RNases using RNeasy?
FAQ ID -942

RNA can efficiently be stabilized in tissue samples by using RNAlater RNA Stabilization Reagent. If the sample is frozen, it should not be allowed to thaw prior to disruption and homogenization in Buffer RLT or QIAzol lysis reagent of the RNeasy Kits.

In addition, you can increase the percentage of beta-Mercaptoethanol added to Buffer RLT from 1% (v/v) to 3-5% (v/v) to enhance denaturation and inactivation of RNases.