Testing an interaction between a continuous predictor and a repeated-measures variable

Question

I'm looking to test an interaction effect between a continuous predictor (C) and a repeated-measures variable with four levels (RM). I ran a repeated-measures ANCOVA in SPSS (putting C as a covariate) to get the C*RM interaction and it's significant. However, there's no way to get the posthoc analyses. Is there a comparable analysis I can run in R to get it? I tried to run a mixed effects models but I'm not sure it's the easiest way to go because I don't have random effects in my model. I also don't know how to interpret the coefficients there (because it gives interactions between C and each levels of RM).

Thanks in advance!