Levelling is a staple of many fanatsy RPG's such as RoleMaster. I also use it with great enjoyment in my current campaign.
But for some campaigns, that may not be the best solution to the matter of experience. Many RPG systems do away with levels entirely, and it works for them. So, if you wanted to houserule RMU into a levelless system, how would you do it? I personally, having thought about this a bit too much, have come to te following conclusion, maybe fitting most for campaigns with a certain horror feel about them:
For levelless RMU, just treat everyone as Level 1, and hand out additional DP's per session (or per time period in the case of NPC's). The learning costs become rather simplified: Only the very first rank in a skill is learned at the lower cost, and all further ranks are learned at the higher cost.
Resistance rolls are treated normally: Characters will be level 1, but poisons, spells and diseases will be of their regular level.
Casting spells "higher than your level" should be treated as normal (with the caster's level always being 1). Thus, magic is inherently unstable and dangerous, and learning a spell list to level 50 will usualily not done for the higher level spells, but for that additional bonus to the spell casting roll.
Under such a levelless setup, campaigns will be more deadly and more cruel, but this can be fitting for some campaigns.
An alternative that is maybe a bit more forgiving would treat all resistance rolls as level 1 on both sides, and would do away with the whole notion of overcasting. This would basically remove levels from the game, and make all dangers based on other stats alone.
One caveat for such a levelless setup would be that it might lead to highly specialized characters... wizards good at two or three spell lists to level 50 even relatively early in the campaign., for example. These are usual outcomes in levelless RPG'sd, and while that may work in some campaigns, it can quickly go on everybody's nerves in another. So handle with care.